that argument stops working when it’s a large portion of a society.

Not the person you responded to, but I'd disagree with that. I'd say that if a large portion of a society can be said to be insane, then that doesn't change the standard for sanity - it just means that the society itself is insane.

Our understandings of right and wrong are somewhat a social construct, and so subject to social change.

Only reasonably within a particular range. There are points beyond which societal notions of right and wrong become self-defeating, and thus irrational at best.

For instance, if one holds that the killing of innocents is such an egregious wrong that it justifies the killing of innocents, then one has created a closed loop in which every purportedly justified killing in turn becomes a wrong that purportedly justifies the next killing, which in turn becomes a wrong that purportedly justifies the next killing, and so on, endlessly.

That's rather obviously irrational at best, and arguably insane, since it justifies that which it condemns and condemns that which it justifies. And that's the case entirely regardless of how many or how few people believe it.