The way I look at it as I look at Naruto. Weird right? Not quite, you see it's like this Ami was always a little bit kind hearted and hopeful. These qualities did spur the foolish dream of Arrmagandon children living in peace with humanity. It's much like the Jinchuuriki in Naruto (and yes I know there are only at most nine of them, but let me get to my point.) they were born with the sole purpose of being the seals of a powerful and godlike being of their universe much like the children here in this little story were born with this "evil" power.
As Dai later contradicted himself, saying that there was no good or evil, only shades of gray. So for him to classify these children as evil was the same way the villagers of the nations in Naruto classified many Jinchuuriki being the incarnates of the tailed beasts themselves.
Granted most turned out to be relatively decent human beings, Naruto being the prime example. However, The Third Mizukage in Naruto proved to be Shia in this and more than likely made the village hidden in the bloody mist the way it was a as form of revenge against 'weaker beings and elite clan members' aka Humanity in this story.
Shia wasn't wrong about humanity, because how could WE classify them as evil but never give them to prove the notion wrong? Sealing them away from the world, experimenting on them, using them. So in the end, are we truly the heroes of this story, is Aoi the hero or the villain? Simply, it's a tale of two wrongs.
and if you've ever been taught anything, two wrongs never make a right.
It's like this, was Shia truly right to kill humanity?
Was humanity right to seal the children away?
so then you must accept the fact that the answers given here is a double negative, meaning that there is no right answer. Because if humanity didn't seal the children what was the idea that the children wouldn't turn on them in the near future?
it's much like this, if there was ever a thin line between good and evil it was showcased here. There really isn't a line, and there really isn't good and evil just gray.
Think about this Naruto fans and fans of this novel of gem. If Aoi had chosen to walk away and let the children have their well earned revenge would've that been so wrong? Just like would it had been wrong for Naruto to go down the dark path?
For there is no wrong, there is no right, this is only the action and reaction. Positivity and negativity.
And at the end of the day, the world was never ready for the children, and the children could never bring themselves to trust humanity. So something had to balance this conundrum out.