Police can do (or not do) as they damn well please, and almost always get away with it. Governments give government agents "immunity" to prevent government being held accountable to the people government is supposed to serve. It's all very convenient.
From government's point of view, the worst possible thing is for people to take justice BACK into their own hands. Doing so highlights the incompetence, the corruption, the malfeasance, the misfeasance of government. It breaks the illusion of government benevolence.
And THAT cannot be tolerated ... the illusion must be maintained at all cost.
Should have heeded the 3s advise: shoot, shovel, shut up.