If it's one of your "most horrible fears" maybe your subconscious is feeding into your fears with the thoughts? Sometimes I wonder what I would do if I "snapped" and can fixate on those thoughts. I sometimes have fleeting thoughts of that nature but if I kept suppressing them, I would have "intrusive" thoughts.... it's just basic human psychology.
However, if you get pleasure, excitement, and/or "urges" to do something, you probably need help.
On a less serious level, I think, "man, I would love to punch that person" quite regularly. I have never punched anyone.