“I haven’t killed anyone!” is a low bar to set. Especially there are few people who haven’t at least idly considered doing it.
I've heard that posited as one of the reasons for the popularity of zombies in films & tv these days. If one imagines being a survivor of a zombie apocalype (one typically doesn't imagine being one of the zombies themselves), it's eventually going to get to the point that you're just going to have to shoot that annoying guy downstairs who always plays his music too loud, since he's joined the crowd of Z's muttering, "Brains...brains!"