The shame is unbearable.
The Sakagawa family name is disgraced.
To think this was happening on my land, the old man thinks, in my fields, under my nose, and I am such a fool as not to have seen it.
It is intolerable.
There is only one way, the old man decides, to redeem the family's honor. He looks around the kitchen to find a suitable knife, then doubts that he has the physical strength to do what is necessary with a knife.
So he takes up the old shotgun, the one he uses against the birds.
It is not ideal, but it will have to do.