Slavery

I think that the original black slaves were extremely docile, and that violence was reintroduced through interbreeding with whites.  If you look at other kinds of livestock, the number of males is kept at a minimum in the herd so that it can be controlled - this is true of cattle, sheep and chickens.  You don't herd rams and bulls unless you want to run into major trouble.  Men are more murderous than either of these animals, and yet we were able to enslave them (we couldn't do so to the Indians).  Logic would dictate that you would kill or at the very least castrate all of the men - actually castration would make much more sense given the males' value at auction.