Re: DND
No wonder. "Evil" parties are somehow always a pest in the eyes of GMs, as they often think only about themselves, and ready to betray the party at the first occasion. While morals would hold back good-neutral aligned characters from killing everyone, what's in for evil characters? Or are you playing a non-evil Necromancer?
Things might be easier if they were compelled to be together for one reason or an another. And I don't mean like they are stronger together, because it will lose it's worth in a few levels, while the players would most likely intrigue against each other on the way. Would fit to a WoD Vampire game, but not really for DnD dungeoneering. One method would be to have players forced to not to harm each other. For example, they are all undead under a Lich King's command, a party of Death Knight, Shadow Priest, Evil Rogue, Necromancer, with some free will, but not enough to go against their lord's will, or each other. Then they could go down in dungeons, and hoard resources for the Dragon Queen Lich King. I totally stole this idea from WoW. xD
Also, most plot hooks and stories, campaign modules would be useless, and putting the GM to have to invent his own, what also puts more tedious task to the wanna-be GMs.