Killing another human being is never justified when you have at your disposal another means to apprehend. Why wasn't he tased first?
I can answer that real easily. When someone is reported to have a lethal weapon, you must be able to respond lethally. The man in this instance, they already knew he had a knife, and so they had to respond with the ability to neutralize the threat, in this case, with their weapons. If you watched the video, you know that the police would not have had time to holster their weapons, pull out their tazer, and taze him.
They gave him ample opportunity to obey their commands. They told him to drop the knife, and they told him to stop moving towards them. He did not comply. When he was close enough to where he could cause lethal damage to them with a quick jump, they had no choice but to shoot him.
He made the decision. It's called Escalation of Force. You make them show whether or not they have hostile intent, which he clearly showed. First, you shout to get them to stop. Failing that, you show intent to shoot, such as saying "I will shoot", or pulling out your weapon if it's not already out. Finally, if the person is still in non-compliance, they have shown that they have hostile intent, and you neutralize the threat with lethal force.
They did everything by the book. This was a textbook case of everything done right.