Aelius's abrupt decision for War was not because of the words he heard from the Thief who managed to survive on the mercy of some Demon but it was a decision he took after considering many possibilities in his mind.
Aelius was well aware that the arrival of creatures from cracks or rifts in the sky was inextricably tied to the appearance of Demons on the southern continent. This indicated that the recently formed landmass on the southern side of the globe, also known as the southern continent, was not there when their ancestors lived and that the demons currently residing there are still relatively new to this world, which will work as an advantage for the humans.
Aelius cannot allow Demons with the ability to possess someone such as a Beast King to exist in this World. As a result, he needed to get rid of them as soon as possible before they figured out anything about the World or other races.
