Why does everyone treat Hawks having always been an assassin as canon? I know he was brought in as a replacement for Nagant but as far as I’m aware there’s no actual proof he killed anyone before twice

You're right! We've never been explicitly told he has a kill-count of anything but 1 (rip Jin). However (honestly you knew this would make me actually write, didn't you?)....

1. The HSPC has changed (somewhat)

It's spelled out to us that Madame Prez wasn't like her predecessor. Her methods weren't as brutal, she was way into a war of information. In some ways, crueler. Kaina wasn't executed - instead she had her hair shorn and was defamed, humiliated.

Nagant assumes Keigo has been used like her. Horikoshi says Madame Prez groomed Keigo from a much earlier age than the middle school-aged Kaina so he had way less ability to leave or question (additionally, he was so sheltered from society and marginalized that he simply would have been incapable until he was an adult).

This is what the story says outright. So, yes, you're right - everything else is speculation. But then the question is why people believe this is canon outside of the typical abysmal literacy found in this fandom?

2. But Hawks being Hawks doesn't Make It Easier

Truth be told, I'm on your side. I used to very much doubt he had much of a body/kill count. I still think it might be single digits if we consider actually murdering someone with his own hands/quirk, though I suspect he might be responsible for deaths in other ways. I would have completely accepted Jin being his first (and only at this point) murder.

So why did I change my mind about this? Simply; Keigo's a fucking freak. I say this with love.

Every so often Keigo says or does something in this manga that both confirms he's kind of insane and in a very different moral space than everyone else, and just off-handedly mentioning he went and, after being subjected to third degree burns and essentially losing limbs, immediately went to eliminate every last sample even after the battle (where he was carried off by Tokoyami mind you) as in....destroying Jin's body or ensuring no one can use it. He's offended when it's clear Dabi got the better of him with this.

Mind you, he's the world champion at repressing his feelings, duh, but the fascinating way he speaks about this (a minute after screaming they NEED TO KILL JIN AGAIN) speaks volumes. Keigo's completely undaunted about handling death and its aftermath. If he's never killed before, he's been certainly trained to in a way that he handles it professionally.

There's one more thing that makes me think Keigo did kill before Jin. We can argue over how much Keigo hesitated killing Jin, but I think it's a point in that he did in how much he ABSOLUTELY does not with All For One.

Like he does not hesitate. Immediately tries to put a feather-knife through his brain. Logically, I mean, I think anyone would try to one-shot AFO because the more time the man that has (until he rewound himself) the more time he has to fuck you up, but still. He tries to stab through his man's head as soon as he gets out of the portal.

Here's Keigo just admitting it, albeit saying he expected it wouldn't work, but really, he's more apologizing he can't immediately kill this man.

No hesitance.

My final piece of evidence is that Keigo is currently walking around Japan in a suit with a katana begging mfers to "try it bitch". Like being quirkless, not a hero, none of that is stopping him if he needs to defend himself. And it's not like he can pin someone away with his feathers. Nor does he have dozens of daggers at his disposal anymore, just one blade. He's the type to try and finish things quickly as the manga has shown time and time again. I really hope no one actually tries to assassinate him because there's an extreme likelihood he'll just decapitate them in the SPC boardroom.

3. Red, Red Hands

To recap, we know Keigo has been trained to kill, in a multitude of ways (and not only with his quirk), and has always seen killing as option/tool he can use. The HSPC might not be as eager to kill as Kaina's era was, but they raised Keigo with the intent to use him to be able to kill people and cover it up. While there's no proof of other murders, there's proof he's been given the training, tools, and expectation to kill. And his attitude towards killing isn't making it seem like he's not done it before. Of course, he's not agonizing over it like Kaina, which makes me think he was used sparingly to kill.

But the other thing to ask is - will Keigo continue to kill (and not like in personal defense) or lead to the deaths of others? He's already set on reforming the Public Safety Commission by allowing for the reform of Villains who cooperate, renaming the Commission to distance itself from solely heroism... We're still a few chapters away of seeing what this new president has in store for society and how he'll distinguish his methods from the people who created him, but we also have two hundred and fifty chapters of him expressing dislike of how he's used, so perhaps it's fair to say he's not continuing the cycle?