Quis Custodiet Ipsos Custodies?

Tragedies can strike anywhere, at any time. There is no real pattern to be discerned from when and where mad gunmen will strike -- saving that they tend to commit their mass shootings on so-called 'gun free zones' -- but when the mad gunman is himself a policeman, we have to ask: who will protect us from the people whom are supposed to protect us?