For me, it doesn't make sense why Blast wants to save him. From what we can tell in the manga, Empty Void was his partner, helping him look for the Cubes before he was corrupted. But was he really?

Think about it, his Ninja Village was really a despicable place, kids being sent there for hellish training for 10 years. Doesn't that sound suspicious? It's weird that Blast wasn't aware of how evil the Village was until now.

And let's not mention, what Empty Void really is in the webcomic.

A Monster ... before or after God?

In reality, the Leader is a 300-year old man who discarded his own humanity for the sake of power. Grafting his own body with other body parts of those he killed. So why would he help Blast find God's cubes?

Simple: he wanted the power for himself.

At some point, Empty Void hit his peak. He can't evolve any further. Then he meets Blast and learns about God, so he pretends to be his ally. But really, he found a way to transcend beyonds humans and monsters.

To the monster who was once Empty Void, power is everything to him.

To be a slave to God is just a stepping stone for world domination.