I agree therapy is a good idea. It sounds like you are haunted by some very painful things. It makes sense that you would feel vengeful. Of course two wrongs don't make a right. Maybe medication could help too.

Sometimes when I'm depressed I have very vivid intrusive thoughts of severe self harm. They are horrible and make me curl up into a ball to protect myself. Luckily these usually only last a week or so, most of the time I don't think those things. I think for me it's just a chemical glitch.

I hope you can get some help from a therapist and psychiatrist. You sound very uncomfortable.