Pre-2e D&D didn't assume this so much. You might have been taking out the slavers in A-series for altruistic reasons, or because doing so promised lots of loot, or because they were competition with what you were doing yourselves.
It was 2e that (sadly) brought in the "PCs are expected to be heroes" vibe as a direct response to the Satanic panic of the day; while at the same time banning classes that might not be so heroic (e.g. Assassin). Stupid move then, stupid move now; and WotC has leaned into it big time.
When I ran some of the Slavers series a while back, one of the main PCs* actually had "Slaver" as her randomly-rolled secondary skill. This gave the party some extra insight into the slavers' operation but also meant it was entirely possible she'd talk the rest of the PCs into taking over the slaving operation, as this character hadn't entirely left her old profession behind...
* - her most famous quote, on the party subduing some bandits: "Stop taking prisoners, start taking inventory".