Sam Harris: The Worst Possible Misery for Everyone.

[A] universal morality can be defined with reference to the negative end of the spectrum of conscious experience: I refer to this extreme as “the worst possible misery for everyone.”
 

Even if each conscious being has a unique nadir on the moral landscape, we can still conceive of a state of the universe in which everyone suffers as much as he or she (or it) possibly can. If you think we cannot say this would be “bad,” then I don’t know what you could mean by the word “bad” (and I don’t think you know what you mean by it either). Once we conceive of “the worst possible misery for everyone,” then we can talk about taking incremental steps toward this abyss: What could it mean for life on earth to get worse for all human beings simultaneously?
 

Notice that this need have nothing to do with people enforcing their culturally conditioned moral precepts. Perhaps a neurotoxic dust could fall to earth from space and make everyone extremely uncomfortable. All we need imagine is a scenario in which everyone loses a little, or a lot, without there being compensatory gains (i.e., no one learns any important lessons, no one profits from others’ losses, etc.). It seems uncontroversial to say that a change that leaves everyone worse off, by any rational standard, can be reasonably called “bad,” if this word is to have any meaning at all.