Thinking about it. People have discussed to death and debated over whether Chara was ‘good’ or ‘evil’, and talked about their feelings on fandom’s early tendency to cast them as a villain. But now I’m wondering why a lot of fanworks cast them as a hyperactive little shit who swears a lot, gets mad easily, argues with everyone, and is sassy as hell. 

Sassy and sarcastic I can see if you’re going by Narra Chara. But rude, hyperactive, yandere-esque, and especially the swear words… not so much? 

And yeah, I could make arguments about how hiding their face paints a very shy and quiet image. But I’m not going to grasp at straws from still images. I’m going to use dialogue very few ‘Chara is evil’ believers are willing to argue DIDN’T come from them. 

In No Mercy, which is the one route basically everyone agrees they narrate, they’re direct and to the point and usually distant and unemotional. They’re not yelling that you’re a fucking donkey for believing you’re above consequences and can take back control, or heartlessly ripping into the monsters for being so foolishly naive and weak, or gloating over their victory. They calmly explain who and what they are, and what they want. The few times they do insult you, they use sophisticated words like ‘perverse sentimentality’. When they dismiss the monsters they’ve met until now, they don’t go out of their way to be really cruel and insulting. They remain direct and to the point, and are usually cold and harsh statements of fact. 

Toriel’s check in No Mercy is ‘Not worth talking to.’ They’re not wrong. In every other route, talking does nothing, only persistently sparing her lets you make progress. 

Papyrus gets ‘Forgettable’. Again, they’re not wrong. Very few monsters really take notice of him, most only remembering him in relation to other people. As ‘Sans’ brother’ or ‘Undyne’s trainee’. 

Monster Kid is just ‘in my way’. Cruel, but objectively not wrong. It’s less of a ponted insult, and more how little they think of them. 

For Undyne, the narrator isn’t even particularly cold or disrespectful. In two out of three possible flavor texts, they refer to Undyne as ‘the heroine’. Quite a compliment, and one that isn’t wrong, considering how she’s standing up to a mass murderer and protects Monster Kid. 

Mettaton, in No Mercy, gets similarly positive or neutral remarks. ‘Dr Alphys’ greatest invention’ and ‘Mettaton NEO blocks your way’. He sure is blocking your way. And most of the Underground sure does know him as Dr Alphys’ greatest invention. I guess it could be read as sarcastic, but with the tone of all these other bits of flavor text, it feels more like a neutral statement…? 

And of course. Sans. He probably has the absolute biggest variety of No Mercy flavor text. ‘The easiest enemy’ could be read as insulting, but this is corrected to a more objective statement of encouragement. ‘He can’t keep dodging forever. Just keep attacking.’ Other bits of flavor text are objective assessments of how tired Sans is, how YOU feel, and how dwelling on the flavor text isn’t a great use of time. At no point do they call him smiley trashbag, that comedian, or…anything really insulting. 

At no point do they call ANYONE anything especially insulting. They’re not like Flowey, calling everyone idiots. If anything, a villainous and murderous Chara comes off as more cold and distant. Even the Save Points don’t tell you ‘5 more idiot babies to slaughter’ or specifically demand that you finish everyone off before moving on. Until you reach Sans, Chara makes no effort to discourage or persuade you. They just state objectively ‘x left’ or ‘but nobody came’. 

They just… don’t care. They don’t care about any of these people anymore, not even enough to waste energy on insulting them. All they care about is reaching ‘the end’. (Or one could argue they’re at least trying to act like they don’t care.)

Chara states that they were confused upon awakening and chose to follow your lead. Personally I’m inclined to believe that they’re not lying. They choose to follow our orders to the utmost, even when they do not understand those orders, even when those orders lead them to the edge of annihilation. 

On a final note. It’s kinda sad how Chara’s systematic completionist mentality is cold and horrifying in No Mercy… but in the Winter Alarm Clock dialogue, it’s a funny anecdote of a more lighthearted time which both Asgore and Toriel remember fondly, and a behavior Asriel still continues to emulate even after he stops idolizing Chara. 

Any trait, when taken to the extreme, can become nightmarish. 

(EDIT: I know you were more talking about Chara in general getting written as an antagonistic rude little shit in fan content whereas I glommed onto Fanon Evil Chara, but I think it's the same root cause of "not the greatest person" means "RUDE and MEAN" and must have been a total condemnation of Chara as a person, and Chara's true personality being seen as being "Like Flowey from the jump".)

It is weird how incredibly unlike even canon Evil Chara fanon Evil Chara (or even Not-Evil-But-Not-Very-Nice Chara) is portrayed. Because yeah, they're usually written act more like Flowey than their own self from canon: rude, insulting, short-tempered and angry, snide, cruel, delighting in others' pain. They'll spout off Floweyisms like it's going out of style, like calling people idiots, Sans is the smiley trashbag, "kill or be killed", etc.

In the game, it's Flowey who delights in interacting with people: he is the one who wants to play with them, squeeze all the entertainment value he can get out of them by seeing everything they can say or do, and kills gleefully, terrorizing and mocking enemies as he captures and kills them for his own fun. Without the compassion and love he's used to, his loneliness and isolation has turned to "socializing" in an incredibly fucked up way.

By comparison, Chara's problem the further into Kill All we go is very much marked by growing disinterest and the letting go of all "sentimentality" they have left, to the point of only categorizing people by how much of a problem they will be to fight (and only evaluating how useful or useless the things you check are to completing the Kill All path). People stop being people. They are "free EXP". They are "the enemy". In game, Evil Chara views killing not as their way of having fun, like Flowey. It's a means to an end: power, destroying the world, and moving on. They become very focused, utilitarian, and goal-oriented, while Flowey is the one enjoying it more for entertainment and novelty.

Flowey twisted his own emotions and became a monster in the other sense of the word. Chara blunts and loses their emotions to become a weapon. I think this comes down to a mixture of their basic personalities (Asriel craves companionship and amusement like any kid to try to recapture how he felt before the tragedy that happened to him, suicidal Chara who was told they were "the future of humans and monsters" needs a purpose to latch onto) and how their respective soullessness affected them (ONE DAY I SWEAR I will write my thesis on human soullessness).

So it is really weird how Chara gets characterized in fan works where they're evil "just cuz", because yeah even in the route where they're definitely evil, they act so incredibly different from how fun-lovin', fun-havin' Flowey and yet creators will take so many cues from him in how they depict Chara: manipulative, viciously gleefully sadistic, resetting to play with people like toys and kill over and over again, cruel and taunting, flings insults and taunts around for extra bullying before the murder, screaming in rage and denial when things don't go as expected. Meanwhile Canon Evil Chara just kinda… hangs out listlessly and hardens more and more into cold purpose.

It's part of a weird pattern I've noticed in general where Evil Chara is portrayed really differently from canon. There is a sense of "Chara is ambiguous anyway so this is how I portray them," but like you said, Kill All Chara is actually very explicitly characterized. They have their own personality, but a lot of creators will make them act like Flowey when they're filling in as villain.

Other examples of Fanon Evil Chara outright flying in the face of canon (bolding for emphasis mine):

"With your guidance. I realized the purpose of my reincarnation. Power." > Chara is the one "guiding" Frisk to kill, an evil spirit either trying to manipulate or badger them into attacking, giving them nightmares and hallucinations, or actively swiping their body to do it themself. Note there is no indication anywhere in the game that we are being tricked or pressured to kill by an unseen force. Chara's words just randomly get flipped on their head to suit the "demonic possession" narrative.

"Interesting. You want to go back. You want to go back to the world you destroyed. It was you who pushed everything to its edge. It was you who led the world to its destruction. But you cannot accept it. You think you are above consequences." > SO MANY TIMES, I have seen this line used against Chara by Frisk, Sans, etc, or even just the narration of the fic itself, in the sense that Chara has escaped justice somehow and arrogantly plays with people's lives without care, or is shifting blame wrongfully off themself (y'all you are the ones who played the game and did it though, like literally, the game is just acknowledging this). They'll be carefree and unrepentent about any of their actions, unlike their canon self clearly viewing it as this thing that Cannot Be Undone.

"But. You and I are not the same, are we? This SOUL resonates with a strange feeling. There is a reason you continue to recreate this world. There is a reason you continue to destroy it. You are wracked with a perverted sentimentality. Hmm. I cannot understand these feelings any more. Despite this. I feel obligated to suggest. Should you choose to create this world once more. Another path would be better suited." > Tying in with the "above consequences" line above. Individual stories and entire AUs (like Killer Sans, Dusttale, and Last Breath) where Chara is the Big Bad doing a laughably absurd number of Kill All runs for the lolz, the tormentor and arch enemy of Sans and the source of everyone's misery. Stabby baby, can't you think of anything else to do?? You really like doing the murder dance for minutes on end and waiting for enemies to poke their heads out, that much? Compared to canon, where Chara will accept it with some grumbling because it really is up to you, but shows no enthusiasm or enjoyment for it and "suggests" something else. (What they're doing in the Soulless Pacifist ending is a whole nother can of beans, but another KA in the Underground is canonically something they wouldn't do if it really were up to them. Which makes it weird that's literally all they do if given the chance in so many fan works.)

It's just funny because… evil advisor, above consequences, killing on loop, literally all of this is Flowey in game too! His opening speech is trying to trick you into thinking LV is how you get stronger, he advises you on how to get to the best ending only to use it for his scheme, his behavior clearly shows he does think of it as a game now and only in True Pacifist does he come back down to Earth, and we know he's reset and killed everyone enough times to have gotten bored with it and let the world finally move on enough to let Frisk fall down.

It's frankly bizarre how much even Evil Chara's canon words and behavior get tossed out the window in favor of Even EVILLER Chara that's just Eviller Flowey. I think it's a combination of people assuming "laughed it off" means Chara was literally a cackling supervillain who poisoned people for funsies and for some reason no one saw anything weird or off about that (lol), from there assuming Asriel is emulating Chara as Flowey and therefore Chara's true personality = Flowey even though we see everything we see from them even in Kill All shows them reacting very differently, and also I think people just want a villain LIKE Flowey because he makes for such a delightful little jerk, but they don't want poor likable lil Asriel to get beat up by Sans. So instead Chara gets his personality grafted onto them, so THEY can be the little sadistic dirtbag above consequences who gets beat up and killed. Huzzah! (Because they're not the greatest person, so that means they were literally Flowey but even worse, right?? Right.)

Just, very selective reading of the characters.