From what I understand if you destroy the Citadel you are supposed to win regardless of what mode you are playing but in Slaughter mode this is impossible:
http://www.flickr.com/photos/26996118@N06/3208162444/sizes/o/
From the above screen shot you can see how little damage I was doing to the Citadel. It seems that the regen on it was so high that not even that massive horde of units could damage it
Winning by the main victory condition for that mode (Demigod kills) wasn't very feasible either. Since everytime I killed the enemy Demigod I would have to wait for him to rez again. Killing him took fairly long as well, even with all of those units, because of the healing building right next to where he spawned. Although eventually I could have won by killing him over and over I think the game would likely have crashed before that point due to how many units I had.