WARNING: The following contains spoilers for Chapter 57, “Eida,” of Boruto: Naruto Next Generations by Masashi Kishimoto, Mikio Ikemoto, Mari Morimoto and Snir Aharon, now available in English from Viz Media.
Throughout Naruto, the titular hero always made it clear he would never kill his allies. Even when Obito, Nagato and especially Sasuke proved they were ready to end the world, the would-be Hokage knew that there was hope for them. He had always had that gift of seeing the good in people, so Naruto could tell that it wasn't too late to save them.
However, in Boruto Chapter 57, Naruto doesn't have that same confidence when it comes to saving his own son. As he tells the Kage council, he is indeed ready to kill Boruto if Momoshiki takes over the boy's body.
The Konoha leader knows how much trouble the Ōtsutsuki can be after fighting Kaguya, Kinshiki and Momoshiki. It recently came to a head with Isshiki's warpath claiming Kurama's life, which has made Naruto especially wary regarding the future. Boruto was possessed by Momoshiki and took out Sasuke's eye, but while he eventually came to his senses and was placed under scientific watch, the council rightfully has plenty of questions regarding whether Boruto will go berserk again.
They know there's a chance Momoshiki could come back stronger than ever if he harnesses not just his Ōtsutsuki powers, but whatever he can from the Uzumaki blood within Boruto too. Given that Isshiki left a Ten Tails somewhere, Momoshiki could use this to become an even greater threat than Ishhiki or Kaguya. The council needs to strategize a preemptive strike to avoid another Infinite Tsukuyomi-level crisis.
Naruto understands everything and, while it pains him, he admits that he is ready to do his duty. It's a job his dad, Minato, did by sacrificing himself and Naruto's mom, Kushina. Tobirama and Hiruzen gave their lives too -- the role of Hokage means personal matters take a back seat. Even the First, Hashirama, had to kill Madara, an Uchiha he hoped would be his brother, just to keep the integrity of the Hidden Village intact.
Kakashi and co. have often warned of such decisions in the past, but little did Naruto know he'd have to make one like this. The scary thing is, while Momoshiki has a powerful host vessel in Boruto, Naruto, without the Nine-Tails Demox Fox, and Sasuke, without his Rinnegan eye, might not be enough to stop him.
As a result, Naruto won't be able to risk waiting on the full Karma mark transformation, meaning he might have to slay his own son in cold blood at the first sign of trouble, as more enemies like Code come enter the fray, which could trigger the worst case scenario. It remains to be seen how Hinata and his family, including Boruto, will react, but even Shikamaru knows desperate times call for desperate, heartbreaking measures when there are no other options.