Thanks for dudes in comments for doing author's work and putting a semblance of reasoning in this cringe episode. I'm not so salty anymore and can even make peace with it. But the mere fact that I need an external help to justify some or others actions, it clearly shows that writing was sloppy...
This is what happens when plot get secondary to cheap dramatic effects. Whole reasoning was "It was long time since duke did something heroic, let him save our poor MC from abusive mother". For that sake mother goes enraged for literally no reason and immediately proceeds to publicly beat up her...
Checked about Kanto massacre on wiki. Japs killed >6000 people for only reason they were korean, literally. Its amazing how racist far east is even now, i guess it was much worse hundred years ago.