In this video, atheist journalist Christopher Hitchens debates religious apologist Dinesh D'Souza. At one point, D'Souza claims that evolution cannot explain good nor evil, and that only the existence of god can.
First I want to say that the god hypothesis is impotent to explain the existence of evil as it is to explain the existence of the universe. If you posit the typical good, powerful, intervening deity, how can there be evil? A typical argument is that god didn't make evil, just free will. If you give a baby a hand grenade and it pulls the pin, who's going to blame the baby? If you give a man a gun and he shoots someone, are you culpable? No. Unless, that is, you know what he's going to do.
Another is that god uses evil to good ends, eventually. "Know why the devil's so angry? Because god keeps using his evil to do good." In other words, you have to break a few eggs to make an omelet. To make the analogy better, you have to kill a chicken if you want to eat it. But there's a difference between killing a chicken as painlessly as possible and cooking it alive. If what appears to be evil is in fact god using complex means to accomplish good ends, surely a god with the three omnis (benevolent, scient, and potent) could achieve its ends without using even apparent evil.
As an amusing note; can an omnipotent, omnibenevolent god create evil? That's a statement of the omni paradox that I'd never considered.
The other solution to the thorny problem of theodicy is to reduce one or more of god's omnis. The existence of evil becomes less thorny, though a god without the omnis is less omnisatisfying. Also, in the face of a non-creative deity, evil can have another source.
The other half of the problem is positing god as the source of good. First we run into Euthyphro's dilemma; is it good because god says it is (in which case good is arbitrary) or is it good absent god's will (in which case, there's some conflict with the omnis). Many scholars have considered it and, to that end, I direct you to the wikipedia page. Suffice it to say, I believe the dilemma, at both ends, renders null the notion that good stems from god. And this one can be resolved by reduction of the omnis. A nonomni'd god, can have an external standard against which it can judge.
The other thing I'd like to point out is that, even in the event of a nonomni'd god, most of what they command is not, in fact, good. A casual reading of most histories demonstrates that they're barbaric, racist, misogynistic, and pretty gosh darn vile all around. In other words, claiming that good stems from god is more or less like claiming that good stems from a 1930s Italian mafioso.
Next I'm going to posit an alternative hypothesis and rebut Mr. D'Souza on the claim that evolution can't explain good or evil.