What causes police brutality? How common is it? (a few questions)?

you cant say please put your hands up when 7 out of 10 crack dealers will stab them. i saw a documentary on undercover african american cops tried to arrest key drug players. they decked everybody. he said that people ***** about his ways but the same people wouldnt do his job for 2 times as much as he gets paid. i beleive. many suspects are just assholes.