The reason is that in a purely materialist universe, immaterial things like good and evil and souls simply do not exist. For scientific atheists like Dawkins, Cho's shooting of all those people can be understood in this way--molecules acting upon molecules.I wonder if he's still in denial about the Muppets being made of foam and fake fur. Or about rainbows being made of light refracting through water molecules. Or about masterpiece paintings being just selectively reflective mixtures of pigments. Or about Okami being nothing but a bunch of 1's and 0's. ...He probably doesn't worry much about that last one, but you get my point.
Put simply, I don't see how understanding something destroys its beauty or value. Yeah, I'm a bunch of little bitty things working together in a complicated collection of interactions. So what? I still like myself and value my existence as a bunch of little bitty things working together in a complicated collection of interactions. Life is cool. I'm sure many of you still value me as a dirty bag of mostly copper. (What? I'm bronze, meatbag.) Or at least I hope you do. I know my family does.
It's attitudes like this guy's that convinces me that people who subscribe to his brand of theism are more "atheistic" than any atheist: Everything is valueless unless some random, undefinable, incomprehensible magic says otherwise. Personhood is meaningless because it's physical. Thought is meaningless because it's physical. Emotion is meaningless because it's physical. Magic is all that matters, and knowledge kills his magic.
I'm so glad I don't subscribe to his worldview. It's the whiny emo "life sucks and then you die" stereotype, except he's probably slapped on the promise of an empty, mindlessly blissful afterlife on there to excuse apathy, inaction, and injustice in the one life we know for sure exists.
Good and evil exist, and this guy's the sort who contributes to the latter. I've got a gut feeling this guy believes God has some inviolate "divine plan" and thus anything that happens, including this horror must be "good" (though you can bet he'll spin, spin, spin). Another excuse for his sort to condone apathy, inaction, and injustice. Don't know about souls (evidence, please?), but consciousness exists. Knowledge isn't going to destroy any of those things, so there's no need to zealously espouse ignorance. Rainbows and Okami Amaterasu are still beautiful. Nothing this guy can say will take that away from us.