The latest episode of Boruto: Naruto Next Generations' never-ending "Mitsuki Disappearance Arc" was titled "Kozuchi's Will" and it filled in a very important piece of backstory that fans have been wondering about, ever since they got the big reveal that Hidden Stone's Third Tsuchikage, Onoki, is actually the mastermind behind the monstrous Akuta monsters, and the synthetic humanoids known as "Fabrications." In "Kozuchi's Will" we get to see firsthand what twisted Onoki from being a hero of the Shinobi Wars to a villainous threat - and it's a pretty compelling story that pulls at the heartstrings.
"Kozuchi's Will" flashes back to the tragic memory of when Onoki tried to settle into the era of peace following the Fourth Shinobi War, living out his days in Hidden Stone, and enjoying the company of his grandson, Kozuchi. Onoki tried to instill the values of Hidden Stone Shinobi in Kozuchi by teaching him the philosophy of "The Heart of Stone" - i.e., a Hidden Stone Shinobi's unbreakable will and sense of bravery. Unfortunately, Kozuchi took that teaching to heart at the wrong moment, as he stood up alone against a gang of robbers attack the village, and was killed while waiting for reinforcements to arrive.
Onoki was there at the moment when Kozuchi died, but that's not what ultimately broke and twisted the old master; it was when Kurotsuchi informed her grandfather of the circumstances of Kozuchi's death, that Onoki realized it was his views that got his grandson killed. In his grief, Onoki decided that Kozuchi's generation had no place on the battlefield, and that there should be some kind of alternative to sending young men off to die. His solution was to create artificial soldiers, who could fight and die so that no young men of Hidden Stone had to. The first Fabrication he created was Ku, who is revealed to essentially be his clone.
With this tragic reveal, the thematic crux of the "Mitsuki Disappearance Arc" is now coming into focus. Onoki tells Boruto the dark story of Kozuchi in order to get Boruto to understand his motivations, and see him as something other than a villain. However, while he's sympathetic to what happened, Boruto raises an important point: his friend Mitsuki, while synthetic, very much seems to have a "will" or a soul - as do the humanoid Fabrications Mitsuki has befriended. If these beings are truly alive, then Onoki is simply condemning others in oder to spare Hidden Stone's young shinobi from violence. It seems pretty clear (after all this time) that the arc will have to end with Onoki either realizing his own error in logic, or pursuing blindly until he dooms himself (or is doomed by his creations). Either way, fans are eager to wrap this arc up as quickly as possible.
You can catch the new episode of Boruto: Naruto Next Generations streaming weekly on Crunchyroll and Hulu.