Once he is no longer an immediate threat, an officer is supposed to stop shooting. If he was falling before the head shots then they weren't justified. You might not agree with that but that's the way it is. Later I'll try to find an article I read explaining it. It's a pretty neutral article that just explains how cops are supposed to handle a situation and decide to use their gun.
If someone is coming at me to do harm, I am going to empty my gun.