Meh...Not excusing the behavior of this guy at all. He was certainly asking for it.
But the officers said he had a knife in an overhand position drawn to attack. I can't see a knife at all in the video. Not saying he doesn't have one, but it wasn't drawn like they described.
Also, they said he was charging at them from 3-4 feet, which would have been legal to use lethal force at that point. He was never closer than like 10 feet from them.
So (surprise) they lied to make it look better on them.
Were they on edge because of the shit that went down in Ferguson? Probably. Was this dude clearly being confrontational and showed potential to get violent? Certainly.
Once again, I see this as a fuck up from all sides.