When people write fanfics, they change the characters sooooooo much that it's annoying. The one who gets on my nerves the most is Sans the skeleton from Undertale.
Seriously, when did he become a sex beast and want to screw an OC? I mean, he may be capable of falling in love, but he'd be subtle with their relationship and offer ketchup or something, but that's it. He probably won't just attack an OC with rape either, but he'd probably just be like "so... heya. we've been dating for a while so... have ya ever thought about... you know. not mandatory or anything, but something to think about"
He's NOT a sex beast! I'm not one of those fans who are in love with him, but I read xReaders sometimes when I'm bored. This isn't Sans! Sans likes ketchup. He's lazy. He'll probably suggest taking a nap for a date.
But in fanfictions all he wants to do is plow.

Same with Bill Cipher, man.

But, I mean, hey; it's a mistake we all do at some point. In some fanfics Mettaton is a Yandere. Flowey is a bit emotional. This is just something all us fanfic writers have to work on. :>