Bester wasn't killed because a) it took a *lot* out of Ironheart to take
out Kelsey, and b) Bester *shot him* prior to being taken down, and the pain
and trauma of being shot has a tendency to have an adverse effect on one; it
took all his strength just to knock Bester down before collapsing to the
floor, his energy (as you can see in the film) fading away....

jms