Hm. Well, there's some false logic in the "the majority of human historical figures are evil by D&D's standards", insofar as D&D's standards simply don't apply to real people. Nobody in history has ever been subject to a Detect Evil spell, so the point is largely meaningless. D&D "evil" does not exist in the real world.
But let us consider...
Slavery is the act of making sentient creatures into property, the literal ownership of another person. The PHB says, "'Evil' implies hurting, oppressing, and killing others." Can't get much more oppressive than outright ownership.
So, slavery is evil, yeah. However, we must remember that a character's alignment is a long-term average of actions. An alignment isn't determined by only one thing the person does - each and every act has a chance to weigh in. So, the slaveowner committs some evil in ownership of another sentient. If that slaveowner is also kind, benevolent, charitable and all to his slaves and everyone else, then he or she will not necessarily be Evil overall. Even Good aligned people can occasionally commit an act that harms others without suddenly becoming Evil.