Why blaming the police is wrong!

We have all seen it, the police do something and the people rise in horror that the police didn't use non-lethal force instead of shooting the person.
Sounds so logical, doesn't it, Why not do something non-lethal  to prevent serious harm to people.

Ok here is the problem in a nut shell.

You are being threatened by someone with a knife or a gun and it seems like a real problem for you.  And then your saviour comes in the form and uniform of a policeman!

Words are exchanged between the parties involved and the policeman can not seem to get the situation under controt.

Now the question I will ask you.  Should the policeman shoot the man threatening you before, or after, the man shoots you?

Oh, I think I know your answer, but is the situation different if the policeman is the one being threatened?

Now it gets a little more difficult. A riot is going on and the mob is throwing Molotov cocktails, should the police shoot the thrower before the Molotov cocktail its you or your children?

Now it gets to the point of interest. At what point do you think the police should protect you from harm, but not protect himself from the danger?