I'm not a lawyer but I believe the following applies
Killer- any one who takes a life could be identified so even iof it was justified by self defense, or say combat operations, though it carries a harsh tone and is often mis-connotated.
A homicide - is a human death caused by other than natural causes or acts of God.
A murder is defined as a killing with intent including out of anger or passion. as opposed to manslaughter which covers many 'accidental' deaths
An assassin - targets an individual, stalks, and kills them. Often for political reasons or monetary gain. Makes killing a business proposition