Update: I thought a lot on the problem, and managed stopping both the murderous fantasies and the need for them. It turns out it was a symbol of me being able to outmatch the worst life has to offer. It wasn't about harming people, but about being able to be on the level with the worst challenges - and ended up turning myself into one. Thankfully, this seems to be over now.
Still, my generic tendency to like violence hasn't vanished. I wish I could figure it out.