because he didn't realize it was reality and not a game
So clinical sociopaths and psychopaths who commit mass murder like Bundy and Dahmer should be forgiven and let go because they mentally didn't process what they were doing was evil.
Got it.
This kid should have been killed on the spot. This is practical, not emotional though. Because no jail exists that can hold him. He exists outside the scope of punishment. And at best, he'll be tortured then burned at the stake for being a bandit. A clean death would have been the "kindest" she could have done "for a fellow Japanese".
But since vast majority of JP are horribly xenophobic including authors (this isn't a stereotype, it's factual; go there as a black person and ride a train) they somehow use weirdo logic to excuse their own human rights violations, without introspection/retrospection -- even in fiction. (I won't cite examples, since they are a bit crass, but IIRC there was that one nationalistic author that went too far and got the LN/Manga/Anime cancelled.)
I despise "allies jumping in the way to save the villain" trope. It's like they completely ignored and forgot Mai-Mai's warning 10s later. This entire arc sort of rubs me the wrong way; I hope the LN did a better job explaining it, and manga just glossed over it for mass-market publication sanitization.