Have you ever had someone run at you with a knife?
I haven't, but I have had someone run at me with a 9" screwdriver, and let me tell you this, there isn't much thinking involved in the reaction. If I had a gun I probobly would have shot him.
I don't much like police, because 95% of the times, they are assholes. I don't think how they react most of the time is acceptable, however, if the case was that the guy was running at the police officer with a knife, and I mean 5-10 ft. away, then yes, fire away.