It is too simple to suggest white or Europeans were in favor of slavery or against it. The abolitionist movement began as early as 1000 AD in Europe and gained momentum after the Reformation primarily in northern European nations and then slowly spread through the rest of Europe and then more slowly to the colonies of European nations and eventually to most of the world.

But the fact, that the abolitionist movement existed, does not mean every white or even black slaveowner supported the abolition of slavery. In fact, resistance outside of Europe to abolish slavery was great even in European colonies, the southern states in America being an example. So even when slavery was outlawed in Europe it was difficult to extend those policies to countries where slavery had become a critical institution in the economy of the colony.

Slavery was supported by both colonists and native peoples in many European colonies and had existed in most cases before Europeans arrived. This was especially true if Africa. The link at the bottom of this post shows when various nations legally outlawed slavery.

One of the major challenges in ending slavery in Africa was several of the dominant African kingdoms that were the "source" of slaves used slaves as a currency to provide them with the means to finance their empires and the Middle East continued to demand slaves well in the 20th century. So even when the ocean route of slave trade was effectively closed down by Europeans, African kingdoms sold their slaves through the overland trade routes. One can argue which was ultimately better for the victims of slavery... but I don't think such arguments are productive.

In that sense, Sowell is actually correct in the greater scheme of things. It is also worth noting, that sadly, the part of African where most of the slaves originated would likely have killed their captives if a market for them had not existed in the form of slaves. That is not to condone slavery, but simply to recognize what the alternative was.

Even today, those nations continue to experience some of the fastest growing populations on the planet. Without slavery, the excess population would have been culled through war and execution. Today, intentional emigration provides an outlet for the surplus population in contrast to death and slavery. When studying history, it is always worth noting the alternative before condemning a practice.

So, while it is easy to condemn Europeans for participating in the slave trade based on modern western European values with respect to slavery, it is more difficult to make the case that they were the reason for slave trade or that absent their participation things would have been different.

In fact, arguably, had Europe not participated in the slave trade and colonization of African, things may have been worse. Again, that does not suggest colonization was good or slavery was good, but it may suggest that there really were no better alternatives in the eyes of African kingdoms, who traded in slaves for thousands of years and whose cultures, economy, and social structure was built around slavery as an institution and source of income. Europeans simply offered these kingdoms a new outlet for their slaves.

https://vividmaps.com/wp-content/uploads/2022/07/Slavery.jpg