Someone who brutally and cruelly forces people into servitude is a slaver. If you come across this word being used as a verb, it has an entirely different meaning (and pronunciation), to slobber or drool.
You can use the historical word slaver to talk about the slave traders who captured and sold human beings into slavery, as well as the slave owners who forced enslaved people to work. It also describes the ships that transported enslaved Africans: "Many people died at sea on the slavers." When used as a verb, slaver is pronounced with a short a and it has a much gentler meaning. Your dog might start to slaver as you pour food into her dish.