First, the deck:
[deck]Creatures: 30
4 Gravecrawler
4 Diregraf Ghoul
3 Deathrite Shaman
4 Lotleth Troll
4 Blood Scrivener
4 Knight of Infamy
4 Geralf's Messenger
3 Varolz, the Scar-Striped
Spells: 10
4 Tragic Slip
3 Rancor
3 Abrupt Decay
Land: 20
4 Overgrown Tomb
4 Woodland Cemetery
2 Golgari Guildgate
4 Cavern of Souls
6 Swamp[/deck]
Next, card choices:
The Zombie shell: Gravecrawler, Diregraf Ghoul, Lotleth Troll, and Geralf's Messenger - basic zombie shell that needs no explaination.
Deathrite Shaman - Reach
+ Lifegain + Reanimation hate = good card
Blood Scrivener - The deck has such a low average cc and so many ways to protect itself from over-extension that this seems like a no-brainer zombie (lulz).
Knight of Infamy - Just a solid dude that's a real good target for Rancor and +1/+1 counters vs a lot of decks.
Varolz, the Scar-Striped - His stats are not exciting and certainly not Aggressive, but his abilities are where he (maybe) earns his place (I'll go into detail later).
Tragic Slip - Kills early dudes and FA by itself and Morbid should be easy to turn on with so many weenie creatures and a sac outlet.
Rancor - Err'body likes having +2 power and Trample.
Abrupt Decay - Seemed like a solid removal spell, could alternatively be Victim of the Night.
Finally, Why Varolz?:
Varolz has the potential to give the deck new life. BG Zombies has been tried and found wanting in the past - it's not fast enough to kill an opponent before they
get online (like Naya Blitz and Gruul Sligh) and it's not big enough to grind out a game once the opponent starts dropping larger threats (like RDW with Hellkites and Dos Rakis), but Varolz can change that.
Varolz gives you a very specific ability: to have relevant late-game threats in your deck without having to actually make any deck concessions for them. What this means is that you have a very low-curve Aggro deck with more threats (because you can afford to run so few lands) that still has the option to attack with creatures that aren't completely out-classed after the 4th turn.

That's the theory, anyway...