Logic dictates that he should be getting weaker but that's not how the divine dungeon works. The energy of the dead will fuel the killer. In a way, it's like the rising momentum of a boulder rolling down a slope. Once it starts moving then it becomes more difficult to stop.
This mechanism was put in place so that the defenders will be able to remain undefeated the more they kill. So if they were one step ahead, they will remain forever ahead. Whoever planned this defense prepared for almost everything. But they didn't expect to start losing from the very start. Their boulder wasn't rolling, it was being pushed back right from the start. Soverick kept pushing them and the army kept giving ground. Nothing worked, nothing could work against the abnormality that was Soverick Ghastorix.