Earlier quoted context omitted.
No, they're pissed off due to whatever in their psychology. However, they're choosing to resort to violence due to lies being spread by Fox News et al. Other people were rioting due to truth about police actions. While all rioting is to be condemned, there isn't some symmetry between a scenario driven by lies and one driven by truth.
MLK said that violence was the language of the unheard. The silenced will turn to violence. That's not surprising in the least.
They are angry because they lost and they are being told that they didn't lose. If you can be told that anything you don't like is the case then you will always be "unheard."
MLK didn't make this statement so that anyone who was unhappy could justify violence.
You can't just live in unreality and pretend you're a victim. Either you can't see it for some reason or you do see it and you're pretending you don't.