> There is a difference between slavery as an institution of unjust forced submission and "slavery" as just punishment for crimes committed.

The difference is solely the standing law. During the nazi regimen (godwin's law, but really) many of the appalling things they did were legal. It was legal to execute, and it was legal to do forced labor.

Slavery was always legal, that was the problem all along! A slave could not physically escape without the fear of unpunished retribution.