Sword of No Self

Why did a doctor who was supposed to save lives and heal the wounded travel to another world and become a swordsman killer? Is it only when the Buddha picks up the butcher's knife that some people will start to repent of their sins? When it is discovered that killing is the only way to save people, the doctor takes up the sword; when sins cannot be forgiven, kind people will also become the god of death. "Now, why did you name it Selfless Sword? It's not domineering, and it doesn't sound nice." "Because without me, it's just a sword." "