No, see we are talking about the moment when the officer had him dead to rights and didn't need to actually shoot and kill him, remember, everyone at that point was already off the bus.
What you are doing is trying to take an earlier situation and mash it to fit in another time, in a completely different situation.
Now, if a cop, or anyone for that matter, had turned around, saw the dude wielding a knife in the back of the bus and was actually putting someone's well being at risk, then it would of been a justified shooting to protect innocent life.
However, those innocent lives were no longer in danger, the threat was over, and the cops had the dude pinned on the bus with guns drawn. At that point, it was completely unnecessary to shoot him at all.