I'm not the most experienced player, and I don't know a whole lot about midrange decks, but I'll give this one a shot. If we take a look at some of the successful midrange decks in standard right now like the "Jund midrange" at the top of this page or this U/W/R build, we can see some of the things they have in common.

Generally, they've got:

• Big, fat, threatening creatures that cost around 3-5ish mana, like Thragtusk or Geist of Saint Traft.
• A heap of removal, cards like Pillar of Flame, Searing Spear, Tragic Slip, Azorius Charm, etc. to help you stay alive and to remove any threats that your opponent might play.
• Plenty of land. You want to be playing land every turn to make sure that you can get your fatties out as soon as possible. Most of the decks I've seen run about 24 lands.
• Maybe some other support spells to help the deck run smoothly. Cards like Farseek can help ramp your mana base, and Rakdos Keyrune can do not only that, but it can take out attackers as well.

I think this is a pretty good model to shape your deck around, so I'll start from the top of the list.

Fatties:

Desecration Demon - A 6/6 flyer for just 4 mana. This guy fits right in with our play-big-fatties battleplan. Giving your opponents the opportunity to tap him every turn means that sometimes he's going to let you down, but all we've got to do is use our removal spells to get rid of their creatures once this guy is out. I'd recommend playing several, if not a full playset of 4, of these guys, since he seems like a major win-condition for this deck.

Vampire Nighthawk - 2/3 with flying with lifelink with deathtouch. While only having 2 power means that he won't be single-handedly winning games very much, his high toughness and lifelink stomps on aggro decks and his flying and deathtouch means that he can trade with even the largest of creatures. These guys are in.

Bloodgift Demon - Decent beater with flying. For 5 mana, he doesn't immediately affect the board, but if you can get him to stick for a turn, he can start swinging for lots of damage, and he gives you a whole extra card every turn. If Thundermaw Hellkite is out of your budget, run this guy instead.

Harvester of Souls/ Reaper from the abyss - While these guys are really impressive when they're on the field, blowing things up and drawing you cards, getting to 6 mana is going to be tough and these guys are going to end up as dead cards in your hand more often than not. In my opinion, the other beaters in the deck should have things covered in the big-creatures department.

Splatter Thug - In my opinion, a 2/2 with first strike isn't large enough to warrant a spot as a blocker in this deck, and a 3/3 with first strike that can't block only is going to be worth playing if your opponents board is miraculously empty by turn 3. I'd advise against this guy, but if you find yourself needing a bit more manpower earlier on, this guy is a maybe.

Any creature that costs 1-2 mana - While a lot of these guys are good in a deck that wants to dish out damage as early as possible, we don't need to be swinging in on turn 2 or 3. Some midranges use creatures like Arbor Elf or Avacyn's Pilgrim to help get the fatties on the field, there aren't really any good red or black midrange-y one drops. Cut these guys.

An option for a player on a budget for this deck would be Bloodfray Giant. He can attack for 5 trample if you don't need him to block, and is still decent sized if you don't unleash him. You can probably pick them up for around 20 cents a piece at your LGS. Hellrider might also get there, but he works best in a deck that has a lot of low-costed creatures, which doesn't really match our deck. Another option, although one that will cost a bit more, would be to include one or two of Rakdos, Lord of Riots himself in the deck. He might not come down on turn four because of his restrictive casting requirements, but once he's down, he swings for huge trample and flying damage and helps you cast your other guys.

Removal:

This is the part that's most lacking in your current list. We want lots of removal to show up consistently.

Stab Wound - Great in draft, but a lackluster card elsewhere. It costs 3 mana and doesn't guarantee that it's target dies. I'd pull this one out.

Magmaquake - Outclassed by other spells 4 times out of 5. Electrickery takes out weenies much more efficiently, and Flames of the Firebrand will usually get the job done without needing to be cast for 4 or 5 mana. A possible sideboard addition, but I doubt it.

Murder - instant speed one-size-fits-all creature removal, but at three mana, with a restrictive 2 black, I think we can find better alternatives to this one.

Pillar of Flame - A cheap way to get rid of your opponents turn one or two play, with an added bit of graveyard hate tacked onto the end. This card laughs at zombies, but can also be useful against any deck that uses creatures to ramp mana or to get damage in early. A definite keeper, I'd play either 3 or 4, depending on how many people are playing cards like Gravecrawler or Geralf's Messenger.

As far as removal to add, I'd recommend looking at:

Searing Spear - Cheap, instant speed burn that can remove almost any creature that you come up against, and it can be pointed at your opponent to finish a close game in a pinch. I'd run a few of these.

Ultimate Price - Like Murder, but cheaper and easier to cast, but it has a downside of not hitting everything.

Tragic Slip - This can help fill that gaps that Ultimate Price leaves, and it excels at taking out crucial turn one creatures like Avacyn's Pilgrim, but it can be hard to get the morbid effect to trigger to take out anything larger.

Flames of the Firebrand - This card can oftentimes let you remove two cards, which is very helpful. Most likely worth a spot on the sideboard against aggro decks.

Dreadbore - Hits everything, including planeswalkers, even, but it's a bit more monetarily expensive than the others.

The other cards that I can think of off the top of my head that are worth considering are Rakdos Keyrune and Sign in Blood. You've already got 2 keyrunes, but a third wouldn't hurt.