In reply to Indy:
I don't have anything to add on the American cop situation but, coming from South Africa, I'd like to explain a few things about the situation, there.
The current rampage of the mob, defacing statues and being generally unruly, is very easy to explain. You have a large portion of the population who are living in poverty, deprived of jobs, education, utilities (electricity and water), sanitation and basic service delivery. The people are very angry.
The cause of their hardship is largely the ruling party, the ANC. They have done nothing to encourage the economy to provide jobs, they have scuppered education and they have put their own aims above the needs of the nation, thus denying the people utilities, sanitation and service delivery. All of the problems have solutions but these are not implemented. For instance, commercial investment is held up by the fact that B.E.E. rules change too frequently so businesses cannot simply right off the cost of empowering local businessmen - they hold off because they can't manage the unquantifiable risk. Power stations would have helped but the tender process resulted in contracts being given to "tenderpreneurs" (the brothers and cousins of the politicians) and so they're all delayed by decades - and they built coal plants, which was a stupid idea, but keeps the coal producers in the loop.
The angry populace cannot blame the ANC. They are the heroes of the struggle, after all. They have to blame someone because it is the african way: find someone to blame. (To be fair, though, they might have a point because this situation was not created by your average farm worker or miner.)
Now, add to this the antics of the ANC themselves. At every political rally, the songs from the power-struggle are still sung, even though they are no longer relevant. Phrases like "kill the boer" and "bring me my machine gun" are dogma. Violence is celebrated. Anybody who has studied history can point to many examples of how the spirit of the mob drives normally peaceful people wild - imagine how effective it is on a nation of war-like tribes that only found a semblance of peace in the last few hundred years!
The ANC are not the sole problem, however. Parties such as Julius Malema's E.F.F. that have splintered from the main ANC are also militant and violent and dangerously misguided. They inflame the people's passion for retribution against the people who created their situation and all of these politicians go so far as to name that person: the white man and the foreigner.
The spark that set it off was the sudden targeting of the Rhodes statue at UCT. I don't know who started that but I can completely understand why it worked. Rhodes wasn't a very good guy, after all, and the statue was an easy target - you can see it and point at it and rally around it and promise the people a humiliating end to it.
The authorities gave the people what they demanded and the lesson of africa was once again reinforced: violence works. Mob rule works. Have a riot, throw some faeces, this is the route to success.