There are few reasons for me. Classic DTF ranking time!
1. Empathy. The thought of personally ending another human's life is just terrible to me. Who am I to take another life?
2. I'm not religious, but on some level, I do believe killing is wrong. Like Adami said, it's like of necessary at times or at least unavoidable, but I'd like to avoid it if at all possible.
3. Any kind of physical trauma kind of grosses me out. If I had to bash someone's head in, I'd probably faint.