Of course the human may have free will, but the majority of people are trained to follow orders from figures of authority. It is improbable that a police sniper would refuse a hit based off of some ethical dilemma, especially when all the information they have on a situation is fed to them by their police colleagues.

Heroes are rare and their presence should not be taken for granted. Counting on a human to do the right thing is almost never the right choice if you can help it.