In one of the early arcs of Mushoku Tensei the moral compass character repeatedly massacres slavers and it's unambiguously correct even from the vapid, amoral shitbag protagonist's perspective. It completely drops that later on as the protagonist literally has no values or beliefs and just wants to be some guy with a quiet life who doesn't do stuff, which is the biggest failing of the later arcs of the LN series.
May I Ask For One Final Thing is technically an isekai, but the protagonist is a local of the world and the first arc revolves around her beating the living snot out of aristocratic slavers.
Roll Over and Die is one of those fucking things instead of an isekai, but opens with the protagonist being sold into slavery and is notably quite negative on the whole institution and the systems that interact with it.