Octavius makes dark instrumental Hip-Hop buisness. But, uh, it's instrumental. DJ Krush too. Less dark, more experimental jazz stylings. I'd try Cannibal Ox and El-P for slightly dark and twisted vocal Hip-Hop. Not as experimental as Dalek, but headed in that slightly industrial direction. Public Enemy too, most people forget how abrasive some of their production is.
Most rappers want beats that showcase their lyrical talents though, so really messed up stuff isn't all that common.
Prefuse 73 is experimental and wierd, but not very dark. Some instrumentals, some really, really cut up vocal work. Maybe some stuff by The Opus? You could try 'The Unthinkable (Venetian Snares Remix)' by Buck 65, though it was just a one off.
Other than that, you could try listening to some grime. It's faster, and the beats can get alot more agressive than most Hip-Hop. Emphasis is still on the MCs' bars though, so it's not that crazy. Dizzee Rascal, Wiley, Newham Generals, Skepta, Jammer, people like that. Quite a different vibe, and you probobly won't understand what the hell they are talking about most of the time, but it's all good. Just remember 'showering' and 'murking' are positive terms and you'll be fine.
Others too, I just don't have my music collection to hand, so we'll have to go with the stuff that came to me.