If we can't expect police officers to not murder people every time they're in a stressful situation, we're in a lot of trouble. Heck, I'm pretty sure we even expect ordinary people to get through stressful situations without murdering anyone.

What are you arguing here? If you're saying that it's not sufficient to throw the guy in jail without an examination of the larger system, you're right. If you're saying he shouldn't be thrown in jail, you're dead wrong. Personal responsibility aside, allowing police officers to kill with impunity does bad things to a society.