I'm not saying that a character can never be taken in a somewhat different direction in a fanfic, but there are limits, I feel. If you turn someone like say, Steve Rogers, into a serial killer (just an example), you have in my opinion gone past that because you have literally written the opposite of who he is. It's just no longer that character, in any sense of the word really.

I feel silly saying this, but it actually really tends to be upset me personally. I know YKINMK, but the first time I came across a fic that turned my favorite character in my current largest fandom pure 'evil,' it literally made me nauseous. I felt goofy over it happening, but... it did.