@cgarison The cop might have been able to stop that man without killing him.
But the cops failure to do so is a matter of training, equipment, and backup. And you're asking him to either put himself or the public at a _lot_ of risk to try to use those options. As you know, it's very easy for someone with a knife to kill someone with a gun at close range.
That cop didn't do anything morally wrong here. He very likely saved his own life, and there's a good chance he saved the life of others.
@pete
The only factor that could have helped the police officer was the short blade length of the pocket knife as it typically takes a 7" fixed blade to maximize the effectiveness of that tool as a weapon.