The first time Sasuke and Kawaki meet, a sword comes out and someone gets cut. But why did Sasuke cut Kawaki’s arm?
Sasuke had to cut Kawaki’s arm in order to protect Amado, a political prisoner working for the Leaf Village. Kawaki enters a fit of rage after it’s suggested that Amado is still working for Isshiki, and Sasuke has to intervene before the prisoner is attacked.
Amado eases the tension in the room by explaining his side of the story.
Kawaki attacks Amado, Sasuke has to cut his arm
Chapter #47 of Boruto: Naruto Next Generations begins with a battle between Jigen (possessed by Isshiki) and Kashin Koji.
Following the battle, Sasuke breaks down his best understanding of the situation. He infers that as long as the Otsutsuki have Karma, they will never perish and keep resurrecting. Currently, Isshiki has Jigen and Kawaki as available vessels to overtake. Their Karma implants make them susceptible to possession.
Sasuke motions to Amado, one of Jigen’s former assistants, to ask if that assessment is correct. He then reiterates that Amado mentioned earlier that if Isshiki were to somehow reincarnate in Kawaki’s body instead of Jigen, it would be the end of the world.
At this moment, Sasuke accuses Amado of allowing Jigen to perish for the sole purpose of Kawaki being the last remaining vessel. Shikamaru, angry at this idea, says “Don’t tell me this whole thing was a setup for Isshiki, from the get-go!?”
Kawaki, hearing this alleged plan, storms into the room where Amado is being held. He unleashes his Karma Arm attack towards Amado, but is interrupted by Sasuke’s blade. The sword tears through Kawaki’s arm, sending him to the ground in pain while a protected Amado looks on.
“Long time no see, Kawaki,” Amado says. “You seem well.”
Kawaki calls Amado an expletive and asks him what he’s plotting. Amado attempts to absolve himself of the treasonous accusations and explain his side of the story.
So that’s the answer. Sasuke had to cut Kawaki’s arm because the boy was attacking Amado, a political prisoner who’s providing intelligence to the Leaf Village. Kawaki enters a fit of rage after it’s suggested that Amado could potentially be working on Isshiki’s behalf.