Excessive force depends on a lot of things. The north Hollywood bank robbers were shot multiple times (11 and 29 times respectively). Stopping the threat is what is most important. If it takes 1 shot or 30, that's what it takes.
As for being unarmed, I'm not so sure that really applies any longer in our society. Depending on the level of martial arts, self defense, MMA, or other type of hand to hand combat training, someone can be "armed" with the ability to cause lethal harm with use of only their body. That information can't be obtained just by looking at someone. After the fact knowledge is always great, but in a situation where someone kept approaching after being issued a warning to stop, I wouldn't assume I am in no danger just because a person doesn't have a visible weapon.
I am not in any way saying what this officer did was justified in any way, because I don't know if he was or not. The investigation hasn't been finished and if the guy is guilty of committing murder, then he needs to be punished to the fullest extent of the law. If it turns out he was justified and is cleared, again, not saying he was, then I expect a lot more than just looting and rioting.
Eh, weren't those bank robbers armed with automatic weapons?
What does that have to do with an unarmed kid being gunned down in the street?