To dream that youve killed someone means that you have unexpressed anger issues. The dream could also indicate the empathy towards particular person, because you havent deal with your issues. If you kill someone, because he tried to do the harm to you, or you tried to escape of being killed, it means that you are feeling insecure about some things in your life, therefore the killing is interpreted only as the self-defense. If you kill someone by accident, it means that you did some harm for someone and feel sorry and guilty about it. You could also be afraid of taking some of the responsibilities because of the harm that could be possibly done.
Read more about dreaming of Kill in other dream meanings interpretations.