If all impossible answers were eliminated, only one truth would remain.
When the Sage of Six Paths had dealt with Hamura Ōtsutsuki, who had been controlled by Kaguya Ōtsutsuki, he had truly killed his own younger brother with his own hands. The resurrection Curse Mark Gamamaru had given him had not been some ordinary Senjutsu at all, but a Senjutsu version of Impure World Reincarnation.
It was a forbidden technique capable of making the dead move again while preserving the greatest possible extent of their strength in life.
That was why Hagoromo Ōtsutsuki had joined forces with the Impure World Reincarnation Hamura to seal Kaguya away. Later, the Ten-Tails had been extracted from Hagoromo's body and split into individual Tailed Beasts. Only then did the two brothers finally realize that they had been manipulated from beginning to end.
Their mother had never intended to destroy the Ninja World.
But by that point, there had been no turning back. Once Kaguya was released, she would inevitably settle the score with both brothers. To protect himself, the Sage of Six Paths could only swallow his bitterness and choose to suppress his mother forever.
At the time, he possessed only half the power of the Six Paths level. He was far from being Kaguya's match, much less capable of taking revenge on the sacred lands. When he thought of the Ōtsutsuki Clan, whom even Kaguya greatly feared, the Sage of Six Paths began using the talismans Gamamaru had left behind to study the true Impure World Reincarnation.
He did this to deal with the Ōtsutsuki Clan that might descend in the future.
In order to create more powerful figures like Hagoromo, the Sage of Six Paths passed the method of refining chakra to the natives of the Ninja World and established the Ninja