Gun Control & RKBA

There are really only two reasons a cop can use lethal force: to end a (reasonably held) threat of imminent death or grave injury on themselves or others, and to prevent a violent felon from fleeing capture.

The cop in Brussels might have been acting on the later principle, but generally speaking, when you're using lethal force it's to disable (up to causing human death) the person so the threat is ended as quickly as possible. People having extreme agitation, drug use, or whatever can cause a lot of harm in a short amount of time.