Couple of facts:
1) majority of slaves that were brought to America were delivered to slave ships by locals. Europeans at that time did not manpower nor were capable of going inland and capturing the millions slaves. This was done by local Kingdoms, often capturing rival tribe members, sending criminals to slavery etc. This was also beneficial for them. Europeans did not get them for free, it is called slave trade for a reason.
2) more or less the same amount of slaves were traded by Arabs on east coast of Africa following same logic.
3) majority of European countries also used slaves well into Middle-ages. At some point it was replaced with serfdom, which was pretty much the same thing, you were basically only allowed to use the stuff you produced you needed to survive, rest went to landlord, you were also not allowed to leave, where you lived. Serfdom in Russia for example was abolished in 1861, in some regions as late as 1890s.
Slavery has been part of human history for majority of its existence, people from all nationalities and races have been slaves.
Humans are by nature disgusting creatures, but this has allowed us also to prosper the way we have. We are also heavily dependent on social interaction and we need to belong somewhere. Individually, a human being is pretty weak, we have survived and became dominant because of our desire and need to form groups. This is engraved into our DNA and this also effects how we behave nowadays.
What is worrying is that we are really not that strong mentally. How we were raised and how our friends and family see the world and what we believe in are formed by mostly our childhood, but as well as major events that have happened during our lifetime. People who had to grow up during war are the biggest advocates towards finding peaceful solutions. Generations pass on that information, but at the same time those people can easily be transformed into vengeful creatures, if their childhood misery, hunger and suffering is channeled towards revenge. Creating a common enemy, a massively simplified figure, who was the main culprit, why you suffered so much or your family. This is called propaganda and in essence it is tool for above average intelligent group of people with hunger for power, to control the masses, who on majority of cases do not bother explaining the complex way how world works. Hunger for power, which is at the same time is driving for behind human success, is nowadays mainly used to gain that power from other humans.
Propaganda nowadays is more powerful than it has ever been due to emergence of internet and social media.
World around us is very complex, human brain is at the same time triggered to behave in the most efficient way. It has helped us, when we were minding our own business in bushes and sudden noise was most probably caused by lurking tiger nearby. Noise - run. It is also reason, why religion has played such a big part in human history. Religion, apart from being way to keep society under control with rules, has been there to give simple answers to complex issues. Child suffered from disease and died - horrible event for any parent. Why did it happen? God is almighty, he had a reason to punish you.
Religion is there also now, but if you look at big picture, religion is really a collection of thoughts, beliefs and answers. Being liberal, is sort of religion in its own sense, just as believing in white people supremacy. Just as humans are different, what we believe in and how we behave is different. Every now and then, those beliefs have clashed big time. It never happens out of blue, but is rather a constantly growing pressure in a society or between societies/countries/believers. Which is why we have seen so many religious wars, revolutions, civil wars in our history.
We have not had a big conflict since WW2, which was direct aftermath of WW1. There are very few people left, who have really seen atrocities of war. It is not the same, that your granddad tells you, how bad it was. If you were not there, you do not really know.
Which brings us current conflicts.
I think the main reason behind it is the way how capitalism (which basically replaced monarchism) works. Richer get richer, poorer get poorer. At some point this will generate too much tension and then outcome is usually revolution, where wealth is redistributed or then wars between countries, where same wealth is then transferred from one country to another. French revolution saw massively rich king and royalty stripped of their wealth by commoners. Same thing happened in Russia. Colonialism saw stronger and better equipped countries getting wealthier while underdeveloped countries (while rich in essence), getting their riches moved away.
In majority of revolutions, winners were not the most popular faction, but the most vocal and violent one. Bolsheviks only had around 20% support in elections before gaining power. Nazis got around 30% of votes in elections, before taking power.
Same thing has happened now as well. After the last financial crises, the income of majority of working class people in US has not improved, all this economical growth, the country has seen, has gone into pockets of superrich. Corona crises saw massive number of working class people getting fired. Killing of Floyd was just a trigger, people are desperate and then it takes only one match to start forest fire.
At the same time, we have seen populists rise from both sides. Does not matter if they are far left or right, their story is the same. There is complex issue (basically capitalism is not really good for commoner), but individually you look at your own wallet. A fraction comes up that gives you a reason that the reason the wallet is empty, is very simple. Jews, black people, white people, deep-state, G.Soros, liberals, racism you name it. Both sides have now also free platform to share their ideas in social media. Add social pressure, family and friends, who most likely are from same background. The more you point at the other side and blame, the more the other side also points at you. Compromises are impossible, as we are talking about principles people have defined their lives around. Sooner or later, both sides make a move and history will tell who will win.
Sad part is that the common person, who hates extremists is the one who suffers most.
Those 70% of people, who did not vote for nazis, got it as bad if not worst as nazis.