I think the OP is getting an unnecessarily hard time here.
There are other threads talking specifically about the situation in the US and the murder of black men and women being basically sanctioned by the authorities, and the protests and riots that result.
That situation is sickening, but It's not wrong to want to also talk more generally about the reasons and psychology behind the violent behaviour of groups during riots, across the globe for a variety of reasons (including but not limited to racism).
I wish I still had my old psychology notes and textbooks from Uni, we had a really interesting semester looking at group behaviours and law breaking and I remember watching a documentary about how peaceful protests turn violent and even otherwise law abiding citizens get drawn in. Sadly, it was so many years ago that I don't remember the details, but I do know that the focus was on riots in Europe, and perhaps Hong Kong rather than the USA.
I'm sure that while it's not always the case, rioters do regularly seem to come from outside the community they are rioting in.
It's easy to organise, now more than ever given the use of social media... I remember after the Scottish independence referendum that there was a 'riot' planned for in Glasgow city centre by some supporters of one side.
I know for certain that there were busses booked to bring people in from outside the city, and I believe that there may have been plans also for a small number of coaches to come up from parts of England.
Thankfully, that riot was a bit of a damp squib... There just wasn't the appetite for that kind of behaviour among the population generally. Most people accepted the ballot result peacefuly even if they disagreed.
If people intent on trouble had been bussed in to join a protest like the ones currently taking place across the US though, I think we would have seen violence and looting here too.