I’m absolutist about these matters, I guess. I think universal rules for police should be:

• If someone is unarmed, you can’t execute him.
• If someone is behaving strangely, even extremely strangely, you can’t execute him.
• If you think that someone might possibly be armed, you can’t execute him.
• If someone refuses to obey your orders, you can’t execute him.
• If someone runs away from you, you can’t execute him.
• If you shoot at anyone in any of the above circumstances, you will be fired. (Maybe prosecuted too, but that's outside the scope of a police department.)
• If you find the above rules unfair, or are unable to follow them, you need to go into another line of work.