Basis of Heartless Summoning decks: Use cards that are good even without the enchantment on the battlefield.
Bloodgift Demon - Provides card advantage + decent flying body
Desecration Demon - Creature control + Big, flying body
Griselbrand - Big bomb
Hellrider - Aggro, Haste Body
Olivia Voldaren - Control
Thundermaw Hellkite - Flying, Haste, and potential creature control
Vampire Nighthawk - Good and powerful 3CMC if we cannot play our Heartless T2
Jump a line by triple spacing. I don't like rakdos for this deck at all. He will be hard to cast and is conditional.
I like Necropolis Regent as a finisher. Triple black usually isn't iffy with 8 dual lands, and you swing hard once she lands; with a lot of flying creatures, you're bound to get something in.
Desecration Demon in a land of tokens is a bit underwhelming.
As for your particular list: It doesn't seem as though you have enough creatures to really take advantage of Hellrider 's ability.
Yeah. This deck runs too low of a CMC creature base for Rakdos to be all that relevant. However, on that not, have you thought about throwing in some larger CMC closers for this deck? There are some beastly creatures, namely Chaos Imps, that would provide that ending note if your creatures end up being too small to kill your opponent or removed. There are also some good options in the undying field for lower CMC or Griselbrand for late game.
Id probably drop the hellriders and think about a way to search for heartless a bit more reliably, how about faithless looting to dig through the deck a bit?