Denmark and Britain did some of the most horrible things. Their forms of slavery was extremely brutal. Danish and British slaveowners seperated families. The Danes were extremely cruel with their slaves in the Danish West Indies, thus justifying a brutal slave uprising against the Danish overseers. So yeah they were brutal.
Unlike the popular image of white people going into the jungle to capture black people, this obviously did not happen. Think about it, a group of white people going into the African jungle. Clearly, long before they spotted any black people, the African tribes would have seen them coming miles away. This means that either they would have stayed to fight or hidden further in the jungle that they would have known a lot better than any white person. If they stayed and fought, they would not be captured because even though the white people may have had guns, they would not have used them. What use is a slave if they are dead. This give the upper hand to the Africans because they would kill to defend their tribe. Remember, they would also be skilled hunters. Odds are, if white people did go through the jungle looking to capture slaves, they would walk out with more of their own dead than slaves. This means that it had to be local people who captured them. Probably as a result of tribal wars and prisoners taken afterward.
When the white colonists came to what is now the U.S., did they try to capture Native Americans as slaves? Of course not, they knew that it would only end in mass numbers of their own being killed in the attempt. Sure, they may have killed the Native Americans but there is no way they could have taken them alive because that would mean that they would have to do that without using weapons.
Black people are the primary human contributor to black people's suffering, just as whites are the primary human contributor to white people's suffering (regardless of the quantities of the two). The majority of slaves caught and sold were captured and in many cases killed by other Africans. The sheer size of the population in Africa means it's more statistically likely that more people died from murder, tribal warfare and abuse inside of Africa before, during and after the slave trade than those that were taken out of Africa.
If we are considering the real culprit of human suffering, it's probably the need to eat, drink and have shelter. Those needs are the primary drivers of the human propensity to abuse one another.
The Danes, the Brits, the French, the Spanish and the Portuguese have been responsible for some rather egregious things. However, as is often forgotten by those blacks who already live in the western world, there are more slaves (most of them black) alive today than there were at the peak of the western slave trade.
In the Islamic Republic of Mauritania, a large share of the population consists of slaves even though slavery was officially abolished a few years ago. In the Middle East, which only formally abolished slavery a few decades ago, hundreds of 'indentured servants' (workers without the right to leave, effectively slaves) have died working on massive building projects in the burning sun.
How about we start tackling those practices instead of dwelling on the past in an increasingly desperate attempt to blame whites for what's wrong in the world?