Race?

When the protests started they were accompanied by a wave of looting and rioting which provided the naysayers ammunition.  They equated the peaceful protestors with essentially out of control animals who didn't have adequate human qualities to actually protest.  They were wrong as we have seen peaceful protests continue after the first wave of violence subsided.

Who was actually creating the mayhem is a bit unclear, possible a criminal element, maybe outside agitators trying to discredit the protests or even possibly some protestors who went off the rails.  Still while increased police activity certainly helped it was largely via self policing that the protests have proceeded in a peaceful manner.

Of course the perception of many white people underscores a basic problem in our society.  The idea that people of a different color are somehow different.  That they don't share the fundamental qualities that make us human.

It is this perception that feeds racism.  Unfortunately history shows us how easy it is to dehumanize others who are somehow different than us.  The Jews in Europe were treated as subhuman for much of history.  We tend to think of the Nazis as unique in their beliefs, but not so much.  The idea that the Aryan race was superior was not a one time thing.  

It helps to maintain control when you create a enemy who is both evil and inferior.  It makes it easier to oppress and/or kill them.  After all they don't have the same feelings you have. Their lives are not as important.

It is so pervasive that many don't even realize they feel this way.  However if you ever ask or think, what do they want, you have isolated "them".  Once you recognize that they are the same as you, you already know what they want.