Leo showed Kai what Aion had done in the past. Every event that Kai had witnessed was filled with destruction, pain, chaos, and more destruction. There was even a smile on his face as he caused the destruction of multiple civilizations.
He scoured the universe to find a piece of the spark so he could have the power to shape it into his own image. He hated how laid back and how hands-off Leo was when it came to managing the universe, so he took matters into his own hands.
Leo didn't stop his son either. In truth, he didn't care. From the time he came into existence up to Aion's rampage, countless worlds and civilizations had risen and fallen, so he wasn't affected by the loss.
With a wave of his hand, he could bring back anyone or anything that he wanted, but he didn't because he saw this as a natural cycle.
