The Supreme Court has ruled that polie are under no obligation to prevent anyone from breaking any law. Their only responsibility is to enforce the law once it has been broken and apprehend the criminal. A cop can literally stand and watch you stab someone a hundred times while he eats a box of donuts and sips at his coffee, as long as he arrests him when he’s done, he has fulfilled his duty and is not subject to any legal recourse for not preventing the death.