Alice, who had not just the blessing of the constellation, but the seed seal on her back that aids her demonic energy growth, as well as her normal stats, had grown to a terrifying extent.
No one could fathom the level of her growth. No one could even rival her. Compared to her, everyone else was like ants.
If ants come together, they could do damage to an elephant, they say, but how much damage could they exactly inflict? It was not noteworthy before they were wiped out by the elephant.
The point was, Alice could destroy the world if she wanted to, and no one would survive; that was why Leo had been keeping a close eye on her demonic energy.
If it grew out of control and consumed her, she would be a force the world could not handle. He was preparing her for the calamity, and not so she could end the world with her own hands before the calamity came.