a threat = an ACT by an agressor that endangers ones life and possibly the lives of others as well
in self defense you are supposed to shoot UNTIL the THREAT is eliminated, not so much the person's life functions. if the suspect dies while in the middle of a threatening act, then so be it. if the suspect does not die but is wounded to the point where he can no longer pose a threat (ie: knocked out unconscious) then there is no reason to kill.
two armed guys walk into a store to rob it. threat.
the 1st guy ran off. threat eliminated.
the other guy was wounded and incapacitated to the point where he could no longer pick up a weapon and use it, nor run off. threat eliminated.
clerk came back, grabbed another gun, and shot the guy 5 more times at point blank range. that is execution.
conclusion: death is a side effect, it can happen, but to intentionally inflict it in the form of a coup de grace is no longer self-defense.
anyway, the guy shouldve just closed the shop doors, locked himself in there, checked to make sure there isnt anyone else trying to rob him, called the cops later on, and if the other guy bled to death so be it.