From the typical dream interpretation of 'Killing'.
"Dreaming that you kill someone indicates that heavy stress may cause you to lose your temper and self-control. Consider the person you have killed and ask yourself if you feel any rage towards him or her in your waking life. You may very well be expressing some anger or hatred toward this person.
Alternatively, you may be trying to kill or put an end to an aspect of yourself that is represented by the person killed. Identify the characteristics of this person and ask yourself how you do not want to be like him or her."