Oh man, "viral transformation" and "horror hunger" are two of my favourites, with a side of literal "he who fights monsters." I love it when protagonists are unwillingly changed into monsterous creatures, and discover they crave the flesh of their closest friends and allies..! Add in some "body horror" or "tomato in the mirror" and I am the happiest camper!

However, I usually only use those tropes for my original fiction - in fanfic I tend to stick to "Face-Heel turn" and "villain has a point," with "not brainwashed" for drama.

Really, I just like good guys going bad~