Did you read Relic's Villains Month book? It fleshes him out quite a bit. He's nowhere near the complexity of say, Sinestro, but he's at least more interesting, and his motivations far more clear than Volthoom. I really felt Volthoom was a weak-sauce villain for the finale of such an epic run.
The problem with Volthoom is that he was only revealed toward the end of the arc. If his motivations and backstory had been integrated with the story during the Third Army arc, for example, then it might have been more effective.
I got the feeling that he could have been an excellent tragic villain that generated a lot of sympathy in the reading if he had been fleshed out a little earlier and some of his backstory with and prior to meeting the guardians had been explained.