What from the perspective of the police officer is reasonable grounds to fear for his life, is going to be a larger number of situations than those where someone is actually, with perfect knowledge, making an attempt on his life. Or put another way, people do things with (e.g.) the intention of injuring/obstructing/etc. and escaping, but that a reasonable person would interpret as being an attack on their life. If you wave a gun in someone's face, they are in their rights to shoot you, even if you had no intention of ever firing it.