The problem here is not that the unstable man posed a threat to the officer. The problem is that the officer could have used a proper solution that would have saved both their lives: one shot in the right spot. That would incapacitate the unstable man's ability to arm himself, while not killing him.
One of the purposes of combat training is so that you don't lose your shit on the field. For instance. Military get it so that they don't accidentally mow down an innocent family with an assault rifle during an adrenaline rush. Cops get it so that they don't shoot bystanders when they are trying to stop a robber running away. So on, and so forth.
It may get more terrifying when shit gets real. But as a representative of the law, there are more dangerous repercussions when you lose your shit. That's exactly why you are trained.