This story is in pretty wide distribution now, but it’s worth a moment of your time anyway.
Police operate this way because, to a first approximation, the individual officers never suffer any real consequences for grotesque abuses of their power and responsibility. This absolutely has to change. People trusted with power — and the lethal means to enforce their wishes — must be held to a much higher standard than private citizens.
Without such consequences, stories like this one and this one will become increasingly commonplace. It is already true that, when interacting with police, you are safest is you assume they are power-mad and dangerous, and behave accordingly.