Good theory, I think he did die with regret, but not the way you're thinking.
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This suggests that in some way, he screwed up in trying to make peace. Maybe he was at one point like his elder son and considered power above all else. He created a world where people kill each other for wealth and power. This reduced deaths but didn't stop war. He chose to make the world peaceful with the might of the 10 tails alone, but later on saw the error of his ways. He entrusted the future to the younger son because he didn't have that darkness.
I guess SO6P had lived long enough to feel regret for his doing. So He passed Stone Tablet as his legacy so that in the future, his successors could continue to maintain peace, not to bring the war again.
Well peace is a mere illusion. There really is no such thing as true peace, simply ceasefires. But beyond that I think the elder son stole the tablet and that is the reason for the creation of the Bijuu, to make it more difficult for the descendant of the elder son who would be mad enough to resurrect the Juubi, just a thought. Great theory.