Why did slavery become more widespread and brutal in 1800's

Because this was a period in which there was a great export of slaves, in addition to the fact that there were great discussions of white men and blacks still fighting for their total freedom.

In general, starting from the history of a large part of these peoples, we can say that there was domestic slavery in Africa, and not a commercial slavery, that is, among several African peoples, the slave was not a commodity, but an arm to more in harvesting, livestock, mining and hunting; an extra warrior in military campaigns.

These African peoples preferred women as slaves, since they were responsible for agriculture and could generate new members for the community. And many of the children born to slave mothers were considered free by the community. The vast majority of African peoples were matrilineal, that is, they organized themselves based on maternal ancestry, starting with the transmission of names and privileges from the mother. In this way, a slave mother could become a political leader in her society, having generated the heir to the local leadership.

In addition, a slave who was faithful to his master could occupy a position of local prestige, including owning his slaves. Thus, not always being a slave was a condition of humiliation and disrespect. Even representing a submission, it was a situation that was often the same as that of other free people.