I don't suppose any plant is really evil or immoral--they're just plants, after all (though some could be put to evil uses). I think people talk about crab grass that way because it is so difficult to get rid of and you can really start to feel like it's a fight and the crabgrass is your adversary (especially after a few hours of hard work pulling it out!)

Although it's not really "evil," crab grass can sometimes be embarrassing. I was embarrassed when it was covering my flower garden and trying to take over my porch last summer, not so much because of the crab grass itself as because of how bad it looked. It made my house look neglected and I was embarrassed whenever someone came to the front door. I'm sure my neighbors are glad to see me attacking it now!