Good practice would require that police may only shoot to kill if the victim poses an immediate threat to the life of police officers or someone else and there is no other means to disable the victim.
In the case of Jean Charles de Menezes (July 2005), armed police assassinated a terrorist suspect who at the point when he was shot presented no immediate threat. Menezes could have been arrested without serious injury to anyone. The police were allowed to get away with the killing which maintained and reinforced a terrible precedent.
It would seem, then, that armed police units have de facto impunity to execute anybody who is or might be armed or whenever the police are told in advance that the victim may be armed. No doubt many police officers will not kill a victim unnecessarily, but if they do they will not be called to book after the event. It is also unfortunately true that too many armed police enjoy killing.