They take that chance everyday. Same with most people. No guarantees in this life. If an individual wants to stay safe and avoid possibly being killed at work, do not join the military, or even be cop. If a person feels the need to shoot someone to death every time their safety or life might possibly be at risk, do not become a cop.
I agree with you that there are no guarantees in life. If you behave in a threatening manner towards a police officer and force that officer to make a split-second, life-or-death decision, well, **it happens.