Apocalymon is practically like the Recycle Bin of the Digital World, containing the data of all dead and unreborn digimon. It's entity is dominated by the most large by data size and hence the most powerful such digimon, i.e. the Dark Masters the Chosen Children thought they had just managed to defeat. Because of this strange existentialism, Apocalymon holds deep grudge against the Children and the Digital World as a whole "system" that doesn't allow everyone to live according to their nature, no matter how corrupted it might be. This actually makes Apocalymon one of the few generic "I want to DESTROY the world!!" -characters out there who actually has understandable motives for that: think of him as a kind of conglomeration of all the animals and humans that have "lost" in the Natural Selection of our world, and imagine how they would feel about such a system.