The simplest method to deal with a matter is to kill them all...
When there's nothing left, what problems can there be?
With the raw strength of his elder brother, they always felt invincible in the world, at least until they met someone stronger; indeed, he was invincible.
Don't underestimate just being at the First Realm. So what about the First Realm? Defeating those of higher levels just the same, they're all as insignificant as chickens and dogs made of clay.
If he had been born in the Demonic Path, or had Brother changed his appearance, he would have become the greatest evil of this world.
But Brother was not like that, his mind was filled with the Righteous Path.
He could kill but chose not to, instead opting to save.
That's difficult.
Mortals are complex too; they're not brutes, just lacking in mana, not much different from the rest.
Crafty minds focused on self-interest are naturally the thoughts one should have.