I think about violence a lot.
Why do you do that?
I have too much time on my hands.
Everyone has violent or inappropriate thoughts from time to time, but if you are thinking about violence all the time then that's not good. Thinking about violence so much may make you actually act on it and hurt someone or lead you to getting injured. You should consider looking into anger management.
I only like thinking about hurting people that irritate me.
If you are only thinking about it, and have no desire or intention to actually hurt people, then that is okay.  However, if these thoughts are becoming intrusive, or if you find that a lot of people irritate you on a daily basis, then you may want to talk about it with someone.  They can help you to more easily cope with these annoyances.