Neither of your examples would be considered murder by those doing them. In the case of slavery you can’t murder property.

A murder is specifically an unjustified killing. And as far as I am aware it is universally considered bad. Which makes sense because murder being considered bad is pretty much baked into the definition.

I can’t think of a modern society that does not condemn going into a random person’s house and killing them. Nor any past ones for that matter.

Now in some backwards ass country they might not consider it murder if that random person was homosexual. Or a killing might be justified in the context of a revolution. Or to bring rain. Or when shooting some guy fleeing with your tv in the back.

You realise that it is exactly what I'm talking about: everything is "subjective" if you look close enough, but only an idiot will think everything is therefore on the same plane.

Sure, "murdering" is not "100% always in all circumstances considered 100% bad" (but I still think that "universal" was not a bad choice, the same way "universal healthcare" does not mean there is no illegibility criteria and therefore exceptions). And of course, there is a lot of discussion to even have on what people mean by murder, but that is, again, missing the point.

But that is totally ridiculous to then pretend that it means that "murdering is bad" has therefore as much legitimacy as "touching your nose with the left hand index finger is bad".

The fact that is subjective does not mean that you cannot say anything anymore. It feels like a common bias in "technology" people: some of them think they are so smart and are condescending to "social science", and yet they are lost on concepts that social science considers as obvious.