[Belthan]

I've played Oblivion with the full spectrum of "alignments", and I have to say you have the most options with a pragmatic (neutral) character as opposed to a character at either of the extreme ends of the good/evil axis. The DB quests are about it for a truly evil character. FG doesn't really care how many people you kill as long as you complete your contracts, so there's some opportunity there for some mayhem. The daedric quests run the gamut from downright noble to missions that would turn a paladin's stomach, so there are a few you'll enjoy there. MG are very goody-two-shoes, as you put it, and the short version is no, there's no way to become a necromancer or use dark magic in the stock game. A couple of the plug-ins from Bethesda also have some treats for evil characters (Mehrune's Razor and The Vile Lair in particular), plus the Shivering Isles expansion pack is much better than the original game in terms of letting you do things your own way whether you are good or evil.

If you're playing on PC, however, there are a large number of mods geared toward evil characters that allow necromancy, add new evil factions and missions, and otherwise let you wreak havok on Cryodiil (I think I've even seen one that lets you help the Mythic Dawn instead of Martin and completely changes the main quest, but I've never played it). See the "mod defs and links" sticky thread for links to other sites that host 3rd-party mods.