This isn't a bad start! I think to make this stronger we should focus on what this deck wants to do, going by your description.
SCAVENGING: You'll want a few scavenge creatures obviously, but also the tools to use them. 4 copies of Dreg Mangler would be a brilliant move, 2-3 Drudge Beetle s would be solid and 2-3 Sluiceway Scorpion are great! You'll also want to put stuff in the grave. I recommend 4 copies of Grisly Salvage , and a Jarad's Orders if you can get one; it's great for searching for your key cards.
BEEFABLE CREATURES: Daggerdrome Imp is a great creature to buff with counters and Wild Beastmaster before she gets them, and three should be a good number. Lotleth Troll is a definite keep. Splinterfright is another fun creature to look into.
ARMY MAKING: To make Wild Beastmaster even more effective, making a token here or there may be a really cool direction you could take here; with lots of tokens, that's a lot of creatures getting big off the Beastmaster!! A Moan of the Unhallowed or two may be cool, as would a Kessig Cagebreakers s if you can find one. Otherwise, check out Awakening Zone ; it ramps your mana or gives you lots of bodies to attack and defend with!
WHAT TO CUT: Deathmark is narrow, so take that out for another Murder or any of the above suggestions. Serpent's Gift and Rites of Reaping are expensive, so they may be better out of the deck. Dead Reveler is nice but not too thrilling here.
MANA: You have two Keyrunes, which is great. See if you can get hold of some Golgari Guildgate s, maybe even 4 of them! They're great for making sure you're not lacking one of your crucial colours!
In my opinion the only good scavenge creatures are Dreg Mangler and Slitherhead . You need 4 of each. Next to make sure you get more bang for your buck I'd make Corpsejack Menace a 4 of. Sluiceway Scorpion also isn't bad for a defender that you can then scavenge so make him a 2 of then make Ambush Viper a 2 of as well for a little combat trick to flash him in as a defender. I would do 4 Murder for removal or maybe 2 and 2 Tragic Slip . You should probably run 2 Splinterfright and 2 Jarad, Golgari Lich Lord as well as 3 Grisly Salvage to fill up your graveyard. Lotleth Troll should be 4 of and maybe 4 Vampire Nighthawk . Things like Aerial Predation should be sideboard for specific threats, not maindeck. I'm not a fan of keyrunes generally. You'd be better off with Arbor Elf . Gravecrawler is also a good option if you control enough zombies.
I'll be honest some of the things these guys are saying are not bad but you need to focus more. Figure out your play style and focus your deck around that to be good around your strengths.
TAKE OUT:just bad: Drudge Beetle , Korozda Monitor (maybe as a scavenge target but Lotleth Troll is tons better), Stonefare Crocodile , Golgari Longlegs (has no effects), Gobbling Ooze , These cards above are the cards that the RTR block Golgari players find kind of embarrassing.
If you are running scavenge Splinterfright is in the best case going to be useless. So if you are winning, which is what we are shooting for, you just drew a dead card. You don't want creature cards to stay in your graveyard.
So you're aggro means you need more Lotleth Troll , Dreg Mangler , Slitherhead , Corpsejack Menace Lots of one offs are bad it means your deck will never get the card it needs when it needs it and it will not win consistently.
Cards to look at:
Deadbridge Goliath , a 5/5 for 4 that scavenges for 6? This is very useful for aggro.
Vorapede , Vigilance, trample and Undying? This is a card that you curve out to. The thing that you use the most mana on should turn the game strongly in your favor.
Primordial Hydra , good to play for even 3 mana, has trample which is good against other smaller aggro decks.
. Enchantment removal for the Blind Obedience
, Oblivion Ring
and others that you are going to run into. A one sided board wipe if all they have is 1 toughness creatures and an anti-boardwipe in one card.
, It has flying, lifelink and deathtouch? Lifelink keeps you in the game longer. Deathtouch is great for defense and offense and flying over their heads is great or this is also anti-flying.
Increasing Savagery buff up Wild Beastmaster or something else
Jarad's Orders it is one of the best tutors in standard. Better if you are playing scavenge
Rancor Trample is what is great here. if the creature dies it goes back to your hand? Awesome.
You want to be fast, very fast. you want lots of removal you want to ramp to get things out that make your opponents die.
Figure out if you are more black or green and plan your land accordingly. aggro is mainly green most of the time. So Arbor Elf is great for those decks.If you want a more black deck or a more unblockable deck check out mine for some ideas: CouterpunchFOR THE SWARM!
Love the concept of using scavenge with the Corpsejack Menace but I feel you may be a little too spread out with this deck. I personally found scavenge prices a bit hefty, and designed Don't Feed the Trolls to be more aggressive with lower mana costs. There might be a few combos in there that could help you out! =)