Here's a recipe for disaster: Take a young man (or woman), raise him on a diet of violence, hype him up on the power of the gun in his holster and the superiority of his uniform, render him woefully ignorant of how to handle a situation without resorting to violence, train him well in military tactics but allow him to be illiterate about the Constitution, and never stress to him that he is to be a peacemaker and a peace-keeper, respectful of and subservient to the taxpayers, who are in fact his masters and employers.
Once you have fully indoctrinated this young man (or woman) on the idea that the police belong to a brotherhood of sorts, with its own honor code and rule of law, then place this person in situations where he will encounter individuals who knowingly or unknowingly challenge his authority, where he may, justifiably or not, feel threatened, and where he will have to decide between firing a weapon or - the more difficult option - adequately investigating a situation to better assess the danger and risk posed to himself and others, and then act on it by defusing the tension or de-escalating the violence.
I'm not talking about a situation so obviously fraught with risk that there is no other option but to shoot, although I am hard-pressed to consider what that might be outside of the sensationalized Hollywood hostage-crisis scenario. I'm talking about the run-of-the mill encounters between police and citizens that occur daily. In an age when police are increasingly militarized, weaponized, and protected by the courts, these once-routine encounters are now inherently dangerous for any civilian unlucky enough to be in the wrong place at the wrong time.