The slavery of millions of people by the National Socialist Germans and Imperial Japanese before and during World War II was the worst, because the intention was to literally work the captives to death. The concentration camps were centers for slave labor, and anyone who couldn't work was disposed of immediately. The V-2 rocket, the first man made object in outer space, was built by slaves from Mittlebrau-Dora camp. The Nazis even enslaved French skilled workers to raise production on the V-2. Japanese slavery was based on racism too, with the Japanese race being supeior to all others, including Koreans and Chinese. All of that happened within human memory, and there is no doubt how bad it was.