I think I have imagined it. I've walked around Monticello, and I've thought about it a lot. You raise all the canards just like a good old abolitionist. But the slave owners couldn't murder their slaves under the laws that existed back then. (Nor would it make any sense to destroy ones property.) It wouldn't really make sense for an owner to whip or otherwise abuse a slave. Since the owner considered the slave as his property, except for the occasional sadist I'm not sure how abusing a slave would enhance the value of the property to the owner. Slaves were fed, clothed, and housed. They were given time off. Sure, families were frequently split apart by sales, but there were families. Think about how things are today for many of their descendants and tell me that some weren't better off and happier than some of their descendants.
You also might read a bit of Exodus (and Numbers). There was a lot of grumbling among the wandering Jews that things were better for them when they were slaves in Egypt.
For the greatness of the life of a coal miner, maybe you could quietly sing a few verses of "16 Tons" to yourself.
As for Fremantle, my understanding is that he was a soldier born into military family. I don't think he was an "aristocrat." Supposedly he was anti-slavery, though I usually take such convenient characterizations with a grain of salt. His observations of the way the slaves he encountered were treated, though, are almost certainly accurate.
ML/NJ
And yet it happened regularly. Here's one account
One young mulatto man, with whom I was well acquainted, was killed by his master in his yard with impunity. I boarded at the same time near the place where this glaring murder was committed, and knew the master well. He had a plantation, on which he enacted, almost daily, cruel barbarities, some of them, I was informed, more terrific, if possible, than death itself. Little notice was taken of this murder, and it all passed off without any action being taken against the murderer.--Rev. Horace Moulton<Here's another
In Goochland County, Virginia, an overseer tied a slave to a tree, flogged him again and again with great severity, then piled brush around him, set it on fire, and burned him to death. The overseer was tried and imprisoned. The whole transaction may be found on the records of the court.--William PoeI can find lots more. By contrast, you'll search long and hard to find an example of a white man convicted of killing a slave. I can only think of one case, and in that one the man, Martin Posey, ordered his slave to help him kill his wife, then killed the slave to silence him, so the conviction was a two-fer with the wife killing.
In fact, in South Carolina, up until 1821, killing a slave or free black couldn't be considered murder. Instead it was termed "the highest species of misdemeanor" and punishable only by a fine.