I think the most messed up thing I have ever done was kill two mice.
I was 17 and still had to visit my biological father every other weekend per the law, about 3 hours away from my mother's house. During one weekend visit, for whatever reason he bought me two pet mice. My father was the sort of person who would treat you like total garbage and abuse you, and then do a random nice thing (in his mind) to make up for it (again, in his mine). So I get two pet mice randomly as a "sorry I terrorized you last weekend" present.
I took the mice back home and bought them a habitat with food and toys and everything. But these mice would fight at night, and they would make the most terrible noises. I could not stand these hostile/pained noises while they were fighting all night, it made me feel absolutely unstable and insane, though I never figured out why. One night my reaction to their fighting was so terrible that I felt like if I did not get rid of these mice immediately I was going to go completely insane.
So I threw them outside, and it was winter time, and a cat killed one of them (I found it), and I am sure the other one perished since there was like 2 feet of snow outside and predators everywhere. I still feel haunted by it to this day whenever I think about it, because in that moment I lost all empathy and essentially murdered some animals over the sounds they were making. Makes scared half to death of myself.