u/CgRazor

The character, under some delusion, perpetrates what seems to them to be a just act of violence, only for the delusion to shatter at some latter stage and them to realise it actually was horrific, unwarranted, and quite possibly hurt their own loved ones.

Also present in the Black Freighter sections of Watchmen, and I'm sure I've seen this several times before, but as I can't find any trope listing for it, I can't find a list.