I had always thought that the true reason Lavos was able to have been destroyed by them was because of all the spiritual power that they had gained from their adventure, and all the people they met. Gaspar was on something like that. It could be like the whole Voldemort/Harry situation. Harry touching Professor Quiggle made him die because Harry's Mom's love was with him, and destroyed Quiggle beacuse he was evil, greedy etc.