First of all, there are more ways than just guns for a criminal adversary to kill someone, including knives, hammers, baseball bats, pounding a person's head into the sidewalk using bare hands only, etc. Secondly, a cursory glance at crime statistics ought to dispossess you of the notion that criminals are only armed every "one in a billion" cases. Thirdly, if you really think this only happens "one in a billion" cases, why would you bother standing up an armed response team at all?