Consider this description of an imaginary species I'll call Homo atrox:
Homo atrox is in most respects similar to Homo sapiens, but with a far more pronounced disposition toward cruelty. In fact, studies show that atroxians who engage in wanton acts of exquisite cruelty lead substantially longer, happier, more productive lives than those who try to abstain from cruelty. 
Such studies have shown that the activity most beneficial to atroxian wellbeing is the venatio. Kittens, puppies, baby seals and other conspicuously "cute," helpless animals are favorite game in these games. For the greatest therapeutic benefit, these animals' tormented howls must be amplified so as to be heard above the roar of the wildly approving crowds that attend these routinely sold-out events
Clearly, the atroxian's blood sport is revolting to most of us; we
think that torturing animals for pleasure and amusement is barbaric.
But however much our disapproval seems moral, it cannot be. The
reason is that realist moral theories (whether secular or
religious) are grounded in the nature of the agent. Here, the nature of
the atroxian is such that acts of ferocious cruelty are proper to him;
it is in his nature to be cruel. Cruelty, therefore, appears to be morally good relative to Homo atrox.
1. If it seems prima facie unlikely to you that such a species could ever evolve in any imaginable world, consider that cruelty might be an adaptation that enhances important pursuits like predation, territory establishment and self-defense.
Follow-up post: Species Relativism and Specific Vagueness