I don't like the idea of killing innocent people or stealing from them, even if it is just a video game. But I really love the idea of stealing and assassinating in general... ^^;
Are there a good variety of rich/mean-hearted/evil people I can steal from? Also, are there lots of bad guys to track down and kill?
My characters always enjoy stealing everything but the proverbial kitchen sink out from under http://www.uesp.net/wiki/Oblivion:Lazare_Milvan in Skingrad. Not only is he rich, he's the most mean-hearted, questless NPC I've met in the game.

He got on my first character's nerves so much, she stole every single item in his house. And then dumped it all in the nearest river. That's become something of a tradition for me since then.
http://www.uesp.net/wiki/Oblivion:Alval_Uvani is another possibility...he spends most of his time traveling, IIRC (in fact sometimes he'll meet an untimely end on the road without any doing on the player's part--he just gets on the wrong side of a minotaur or something), so his home in Leyawiin is easy pickings.
As for other rich and mean people you'd like to steal from...I say just play your way through some of the game's quests, and do a bit of exploring. The bad apples tend to sort themselves out soon enough.