Its not understandable at all.
If it were a white guy murdering a black kid in open view like that, the deranged blacks would be out rioting, looting and burning as usual.
It's pretty universal. The Irish stuck together back in the early days in Boston. The Scots stuck together in western North Carolina. And so on.
But that can't be the end point. People have an obligation to get beyond that knee-jerk reaction, study the circumstances, and make a fair judgment.