As you might already know, Bartolomé de las Casas was a Spanish writer, priest, and missionary. As a chaplain, he took part in Spain's conquest of the New World. He witnessed Spain’s and Columbus’s attempts to colonize the land that they discovered. He also saw what Columbus and the other Europeans did to the people already living on the land.

In vivid detail, de las Casas describes the colonizers forcing their way into the settlements of the Indigenous people and “slaughtering everyone they found there, including small children, old men, pregnant women, and even women who had just given birth.”

The records of de las Casas could be considered trustworthy as they match with other historical accounts of what happened to the Ingenious people. These accounts include Roxanne Dunbar-Ortiz’s An Indigenous Peoples’ History of the United States and Dee Brown’s Bury My Heart at Wounded Knee. These histories were written by trustworthy scholars, so de las Casas finds himself in credible company.

However, there are some details that might make you think twice about de las Casas’s accounts. You could cite de las Casas willing participation in Spain’s expeditions as a reason to doubt his trustworthiness. If these events were as atrocious as de las Casas says, and if de las Casas was as appalled as he says, it might strike some as a bit off that de las Casas continued to maintain a role in the horrific ventures.

Another reason to doubt de las Casas’s trustworthiness is that de las Casas still believed Spain should colonize the lands. Again, if the colonizers were behaving as barbaric as de las Casas says, it’s suspicious that de las Casas still thought it was a sound idea for colonizers to live among the Indigenous people.

Las Casas was one of the few sources that reported directly on the atrocities that Spanish colonialism was inflicting (though it is far from the only source). It is important to note that many critiques of Las Casas find that he doesn't deemphasize the importance of colonialism, even when he is explicitly arguing for mitigations against the toll it took on native populations. His argument was not necessarily opposed to colonialism itself: it was against certain methods of colonization. Still, Las Casas is a verifiably reliable source regarding the reality of the treatment of natives under Spanish rule. This caused him to come into direct conflict with Spanish colonial rulers, although this conflict never changed his narrative.

It is important to note that many historians question the numbers that Las Casas gives, but ultimately his descriptions of events are not disputed. This is because, at the time Las Casas was writing, exaggerations were the expected norm in works such as his and thus they do not make his works any less valid than any other historical records from this period.

Considering that Bartolomé de Las Casas's accounts of the treatment of indigenous people of the Americas aligns with the fact that roughly 90% of indigenous people died as a result of European colonization should certainly be a strong indication of his account's truthfulness. While many indigenous people died from the spreading of European diseases, Spanish armies continued to conquer and colonize indigenous peoples and indigenous lands; during this period, the death toll that this took on native populations is considered to be one of the worst population declines in human history.

When Columbus first arrived in the Caribbean, he immediately wrote a letter stating that the indigenous people he met were kind and welcoming and that it would be incredibly easy to enslave all of them. The accounts of mistreatment absolutely line up with the reality of enslavement and mass murders that the Spanish committed.