I looked up 'Murder' in the Dream Dictionary (see buttons above if you haven't already) and found there was nothing in there for that, so I added this:

Murder

If you murder someone in a dream, it is more than likely that the murdered person represents an aspect of yourself that you’d like to see the back of, or perhaps you want to end a relationship of some kind, either personal or professional. Also, the dream could be an indication of childish wish-fulfilment; an unresolved feeling of envy or jealousy left over from your childhood. If you are a witness to a killing, you may be reflecting on changes going on around you that you don't particularly like.

See if that helps any?