He removed his weapons on purpose for the purposes of talking down the man with the knife.

Your explanation of a difference doesn't make sense dude. I think what is not clicking for you is that the guy in the car was not given the chance to do that. Again, he was not a criminal. The guy was experiencing a mental break and asked for help, yet he has someone standing on the front of his car with his gun out and surrounded by more armed officers. It was immediate escalation from the get go on the officer's part. Someone with a clear mental state would have dropped it. Someone undergoing a crisis is not thinking rationally and this guy had a death sentence the moment the cops showed up.