I want to fantasize about murdering this person with a sawed-off shotgun
Can you tell me why you want to murder someone?
Because they are really shitty to me.
We can fantasize about whatever we want, but to the point of murder is disgusting and psychopathic. You need to seek professional help if you want to kill someone, because that is not normal.
I'm not a psycho. I'll never kill someone. I just want to fantasize.
It's still not healthy behavior to do so. You should instead talk to the person so they can stop being rude to you instead. That way you aren't using your fantasies as an outlet.