Once Sarutobi Hiruzen reached his conclusion, he began piecing together the causes and consequences of the whole affair.
After the Second Shinobi World War, the major ninja villages had taken turns ravaging Hanzo of the Salamander's territory until it lay in ruins. He had lost any ambition to vie for supremacy.
Rather than expanding Amegakure's territory and developing the village, Hanzo of the Salamander cared more about protecting his own turf and maintaining his rule over it.
Sarutobi Hiruzen knew that much.
Otherwise, Hanzo of the Salamander would have killed the Sannin when he had the chance, instead of sparing them and lending them his reputation.
Hanzo of the Salamander was a spent force now.
So why was he stepping forward to connect the three members of the Akatsuki with Konoha?
Sarutobi Hiruzen believed Hanzo of the Salamander must have recognized the Akatsuki's potential and feared it might threaten his rule over Amegakure.
After all, they had once been taught by Jiraiya.
That was why Hanzo had used this approach to draw the three in, bring the Akatsuki under his command, and gradually absorb it.
Without a god's-eye view, Hiruzen's deduction was accurate and perfectly logical. It matched the attitude Hanzo had conveyed to the three.
Once he understood Hanzo's intentions, the rest was simple. He would cooperate.
What? Yahiko and the others were Jiraiya's students, so why wasn't Hiruzen looking out for them?
Looking out for them, my ass. Amegakure was hardly a death trap. The village was barely hanging on, and Hanzo had lost his ambition.
Joining Amegakure would actually make Yahiko and the others safer.
What? Nagato had the Rinnegan, and Konan had Paper Release. Why not recruit them to Konoha?
Well... for one thing, Sarutobi Hiruzen chose to trust Jiraiya.
Jiraiya hadn't brought the three children