you have to make sure that your teammates tag doesnt become the red X saying hes dead. it changed right before you got the last shot off to kill the assassinator. if you shot meleed him you would have gotten the showstopper.
It looks like you didn't even get credit for the kill at all...in fact, no message came up saying anybody killed him...strange.
I agree with Brick...he was point-blank range, shotgunning somebody in the upper chest...even if he was a little too far away, the fact is he shot him twice and still didn't even get a kill from it!