Ofcoz they were both worse incidents in human history. The slavery went on much longer, and probably more people were enslaved than died in the holocaust.
However, at least the slavers were not trying to eliminate the black race. They enslaved some black people, but they didn't want to kill them, ilikuwa ni kwa faida yao to keep them alive.
It was a horrible thing to do, but slaves did not experience the horror of being puting on those consentration camps that can killed 10 thousand Jews in one day. They also starved, were burned alive, were victims of crazy test medical experiment, gassed, shot, raped,march to death and the list goes on,
Some slaves were better treated than others, and although it is true that they were at the mercy of their owners, the fact is that most owners wanted their slaves alive, not dead.
The Holocaust was a delibrate and coldly caluclated attempt to completely eliminate certain groups of people, particularly the Jews, but many others were targeted as well. There was no hope for those who were sent to the concentration camps, their only purpose was to die.
ingawaje wakati mwingine Tortures is worse than killing, and i dont means that slavery is not worse, lakini nafikiri holocaust is more worse