There are several very good candidates from other quest lines, so a little patience and creativity is all you need. We want you to give in to the dark side, but we want you to do it on someone that you're going to have to kill anyway, who isn't a publicly known bad guy and who won't be screaming for the guards when you try to take him out (or at least not much). And who knows? Perhaps that non-bad guy that you just whacked might be helpful later on. You have to find a non-bad guy and kill them.

The problem is that critters don't count and the assorted Bandits, Marauders, Necromancers and other 'bad guy' NPCs are exactly that: bad guys. So what's the big deal? You're going to be killing stuff throughout the game, after all. That's the key to joining - you have to murder someone and then rest. Shortly after you begin the game you'll hear rumors about the Dark Brotherhood and how, when you murder someone, the Dark Brotherhood will visit you in your sleep. Beginners Guide to Oblivion: Dark Brotherhood

Oblivion dark brotherhood armor