What does it mean when someone you know kills you in a dream?

its probably nothing bad. i think it means you never passed on a chain mail or you have watched a scary movie recently and some one died in it. So it might be you read the same thing or something it is still on your mind so you dream about it.

A different answer:

Characters in dreams often represent different aspects of the dreamer's personality. So the person you know in the dream could very well represent a part of yourself. For example, if the killer is someone from your work, the dream might be trying to say that your work is "killing you" in the sense of wearing you out, or taking up so much of your time that other parts of your life, such as family or social life, are "dying." Alternatively, if the killer in the dream is a bartender from your favorite pub, the dream might be warning you that too much drinking is "killing" you.