The funny thing about “classic“ horror fiction is that later folks attempt to add depth to the characters, but in doing so they actually kind of deviate from why the characters were effective movies and villains in the first place?
Like, having Victor Frankenstein motivated by purely good intentions instead of his own egomania, or having Henry Jekyll be a good person corrupted by his dark side rather than a Bad Person looking to do Bad Things without suffering the consequences, or claiming that Dracula was a good person driven to evil by fate and spending the intervening centuries looking for love rather than a literal baby-eating monster who literally treats women as things to possess or as things “belonging” to other men, for three of the most common reinterpretations I’ve seen.
Wanting to put your own spin on works is great, it’s just weird when the work portrays assholes as assholes, and then other writers spending the next couple of hundred or so years trying to rehabilitate said assholes.
I mean, if you wanted characters in the text to expand upon to make them more fleshed out, folk like Frankenstein’s Elizabeth Lavenza and Dracula’s Lucy Westernra are right there! Heck, the… questionable film Mary Reilly at least took the idea of adapting the Strange Case of Dr Jekyll and Mr Hyde and telling it from the perspective of Jekyll’s servants (whose perspective of the events of the book aren’t covered in the novel despite their being present in the background of a lot of the action)… Admittedly they decided to do it in the trite way of making the titular Mary Reilly (an Irish maid in Jekyll’s household) end up in a kind of romantic relationship with the doctor (romantic subplots being another addition to the story many writers make), but the idea is sound.
Heck, even without getting into the EXTENSIVE notes Bram Stoker left behind about the production of Dracula which included a lot of characters that didn’t make it into the final drafts (such as journalist Kate Reed, a friend of Mina Murray and Lucy Westernra who later was used by Kim Newman in his Anno Dracula books), you could easily tell an effective horror story from the perspective of, say, those poor London kids that vampirised Lucy is eating prior to van Helsing and Co. realising what she’s up to.
Could probably get some kind of class commentary about the rich coming to eat the poor, for example.
There are ways to fleshing out and expanding up on the material in ways that don’t diminish the horror the characters are going through, if that makes sense? Plus a lot of the time, the attempts at addiing sympathetic elements to the story are kind of at odd with the scary parts, such as the Francis Ford Coppola adaptation which portrayed Dracula as both a sensitive romantic… and a man who both feeds babies to his wives and ate Mina’s best friend to make himself young and hot enough to seduce her (Mina).