As noted by Reynardine in #1401061, European slaving was conducted in very much the same manner, the only significant difference being that it was called "serfery" and not "slavery".
Manor Lords had custom and the divine authority of the Catholic Church to treat the serfs in any way necessary to exact work production from them and fill the tithing vessels of the Church. The Manor Lord also had the right to have sex with any serf, male or female, regardless of age, including "le droit du seigneur" to have "first crack" (yeah, that's where the expression comes from) at a newly-wed female serf before the honeymoon could officially start. This was done "to improve the breed" of the serfs.
Not "exclusively American".
For "the vilest", you would have to go to Muslim slavers within the Islam Empire roughly from the 8th Century to the early 20th Century. It was illegal in Muslim communities to keep a black African male slave unless he was castrated which was usually done shortly after capture or purchase to all black African males regardless of age. Castration methods usually yielded a fatality rate between 80% to 90%, but made the victims easier to handle and eliminated most of the problem of black African babies being born. Babies born to black African women (black, white, or mixed) were promptly taken from their mothers and killed by live burial (so that Allah could get the actual credit for the death).
The main reason for universal castration and universal infanticide of Africans was to make very certain that the pure "white" blood of the Arabic race would stand no chance of pollution. It didn't matter to the Arabs if they had a high fatality rate among slaves, new ones could be captured and beat into submission more cheaply than the cost of raising and training an infant through childhood until it could be of real economic use at about the age of six or eight.
I can imagine very few fates more horrifying and vile than to be a black African captured and castrated by Arabic slavers and then worked to death, unless it would be that of a black African woman or girl taken in the same manner, raped on a regular basis for years, and to have all the resulting babies killed as they are born.