There are natural reasons for morals, such as explained in game theory. If you really want to understand how morals can come about through evolution, then read on... and if you don't or you simply want to deny it, then go ahead and skip the rest of this.
Humans benefit from playing "tit for tat", where one person helps out another in an equal exchange, much like how modern economies work. But it's not always possible to return favors, especially in societies of numerous people and in exchanges that aren't readily weighed or valued... so we have the "free-rider" problem where a person gets favors from others and doesn't return them. To counter this, people have reputations based on their good acts towards others, and that reputation is spread through gossip. If you're a cruel person, then people will avoid you and avoid doing favors for you because there's nothing in it for them.
So what determines "cruelty"? Well, most of the things that are generally seen as "evil" are defined that way because they're things that we want to avoid. We value the truth, so we hate when people lie to us. We value our possessions, so we hate theft. We value our lives, so we hate murderers and other violent people. If you can't see why death is seen as something to be avoid, something more than "just nature taking her course", then you're not even attempting to project your own feelings onto others -- we all have these same feelings, even those of us who don't follow a holy book.
But sometimes things are just cultural taboos. We're against things like human sacrifice because we value human lives, even though we weren't always against that... in fact, the New Testament talks about killing an innocent virgin for the sake of appeasing God's wrath in the form of Jesus. But nowadays the enlightened folk try to avoid such taboos, and we wish you'd stop going on about how great human sacrifice is.
