I play D&D on a regular basis, and I love it. I don't think there's anything wrong with it, but I can easily see how some Christians might have a problem with it. It contains spells and magic, which I understand is a sin for Christians.
There are monsters, demons, etc. Of course, if you're playing a good character, you'll be fighting and defeating these demons (and isn't that a good thing?), but if you play an evil character, you're probably siding with them. Of course, if this is a problem, the solution is to simply not play an evil character. Be a lawful good paladin instead.
Some people, none of whom have ever actually played the game, say it's bad because the spells are real, and that anyone who plays it is unwittingly dabbling in the occult. That's just silliness. If that were true, I'd have an enchanted sword and a giant tiger to do my bidding, and people would be getting Magic Missiled left and right.

No, the spells are 100% fake.
In some games, your character can choose to be devoted to a particular deity. I can see how that might be worrisome for some Christians. The deities, however, are entirely made-up, and of course there's no real worshipping going on. Anyway, I've never actually played in a game where deities have been an important factor.
The great thing about D&D is that it's easily customizable, so anyone can enjoy it. If a devout Christian wanted to play, but was put off by the different deities in the game, they could modify the rules and create a campaign in which there is only one deity, Yahweh/Jehovah. Or they could just leave out the deities entirely. Anything's possible.
So, no, D&D is not evil. It's what you make of it.