Nice use of the defiler. You might want to include some eldrazi, which could easily be cast for cheap. I would recommend taking out Subversion because it is a bit too expensive for not much effect. Dreadbore and Terminate are really good R B removal spells. Purphoros, God of the Forge can make creatures cheaper the more you cast on one turn. Mark of the Vampire can be very good for life gain especially on Rakdos, an 8/8 flying, trample, lifelink that makes your guys cost next to nothing is no joke. Sign in Blood can be used for card draw, or to trigger rakdos, Toil / Trouble has a similar effect and is really good against most blue decks.

What about Gut Shot for when you have to play him that turn