Guess I got spoiled by reading Kant in one of my college classes. He was all about trying to describe morals based on rules.
That said, picture someone killing a kitten and someone else killing a cockroach. One could prevaricate all day with highbrow reasons for why the first is monstrous and the second is perfectly moral -- but honestly, for most people, I suspect it comes down to the fact that kittens are cute and cockroaches aren't.