overseers actually carried out extraordinary violence to fellow slaves

It would be pretty unusual for a slave to be an overseer. It's a little out of my area of expertise, so can't say that it never happened, but it would be very uncommon. Instead, I think what you are talking about it what would be called the "driver"-- which is of course the source of the modern term "slave driver".

The overseer was a hired hand... his job was to get as much work as possible out of the slave. Contrary to what you see in the movies, it was usually not in the best interest of the overseer to be unusually cruel to the slaves. Of course, that didn't mean they were kind either - only that the ruthless beatings you see depicted in Hollywood would have been pretty unusual, as the owner wouldn't want harm or permanent damage coming to their investment (rendering them unable to work).

Cruelty came in all forms; Frederick Douglass' autobiographies' documented the systematic violence that he and many other enslaved people suffered to break down their resistance. Harriet Tubman suffered permanent brain damage from a lead weight that a slaveholder threw at her head and that nearly killed her. Harriet Jacobs' narrative details the sexual violence many enslaved women faced. Charles Ball's narrative contains numerous grotesque abuses that he witnessed or suffered. And these are just published narratives from the daring and lucky enslaved people who reached freedom. The non-narrative testimony of the 419 people unearthed from the African Burial Ground in New York City has been read from their bones. In addition to one woman who appeared to have been tortured and killed (her wrists had been beaten to the point that the bones were basically just powder; her ribs contained musket balls) the vast majority left behind skeletons that indicated malnutrition and severe overwork, including the 40 percent of burials who died before the age of 12, whose work life was likely a contributing factor to their untimely demise (most gruesomely, an eight year old boy, whose skull appeared to have collapsed from repeatedly carrying heavy burdens on his head). And of course New York was also home to two mass incidents of public violence during its slave revolts in 1712, when approximately 20 people were publicly tortured to death (mostly they were burnt alive, but others were broken on the rack and drawn and quartered) and in 1741 when an orgy of public burnings took place in response to the most likely non-existent slave revolt the New York authorities investigated that year. Violence was also not just limited to the workplace; In "Out of the house of bondage," Thavolia Glymph details the relationship between enslaved people and their mistresses, detailing the "whipping frenzies," that slaveholding women inflicted on their human property, partially to assert their own superiority in a hyper-masculinized society that left them with few outlets to express control and fury. While enslaved people did inflict brutal violence against other enslaved people in the workplace it was as a proxy for their slaveholder. They didn't have a choice and their refusal might result in their own brutalization, so excepting a few notable sadistic baddies, we really should recognize this violence as originating with the slaveholders and acknowledge its pervasiveness as one of the many evils of the peculiar institution