The word he uses throughout the series to describe his power, kenzoku, isn't translated as slavery though. It's kin and kinship, with those vassalized through his power becoming part of his household (making him stronger as a result) and he considers them family (Ash explicitly explained it this way the latest raw).
Similar wording or feel to when a Vampire that expands their brood and calls their thralls their family (through willing or forceful bites), the original Ash here is saying to expand his family/clan/household even if he has to rape them.
Obviously he mostly enslaves them, either willingly or through sexual assault, but the author is trying to flower up the language and call it differently so we'll leave Ash and the author to try and justify themselves later.
Maybe use subjugation or enthrallment?