There's two things I guess:

1. Did you recently watch anything on TV or in the cinema?

Dreams are very strange things, and what we watch and do affect us greatly. I still remember once after watching resident evil movie (or playing the game, can't quite remember) that i had a dream where i was going to school and then all hell broke loose with zombies and everything.

So just wanna say that if you did watch do something which resulted in the dream, then it's fine, just give it some time to pass.

2. Do you know the person that you "killed" in your dream?

As I said above, it could be that you know this person and he's done something to you that hurt you (mentally or physically). Subconciously you will hate him and therefore in your dream you might want to harm the person. In this case, you probably should tell someone about it. Sometimes that's all that's needed - getting it off your chest and you will be fine. Talking to someone you don't know actually works surprisingly very well.

If it does further affect you, maybe you would like to try some help.