maybe this is just the classical moral objectivist vs relativist argument but if you're a relativist the idea that morals are objective is already something you disagree with, and the idea that morals have a truth value is likewise inconsistent
being a relativist doesn't justify immoral actions, it just attributes them to something other than the universe. morals are ideals that require some kind of application to be effective. the very fact that people *can* disagree on what is moral is enough to disprove the idea that they are universal (if they were truly universal they would be self-evident and not require agreement)
this isn't even a marxist thing imo, it's just the realization that you aren't the universe (because it's pretty curious how the objective moral truth just so happens to be the thing i agree with)
