The big issue is that those officers have a duty to act, to stop a knife wielding crazy person from walking off. Regardless if you agree with tactics, or outcome, they have a sworn duty to stop that person. If an officer feels the life of himself or others are in jeopardy, the use of deadly force is always justified.
I'm not going to say the officer was wrong or right, I wasn't there, can't see what he saw, understand what he felt, or why he concluded that deadly force was necessary, and that's why we have jury trials and not public opinion trials.