Respectfully, I would expect CIA operatives to flush after themselves and not leave their DNA around the house.
I don’t know about Japanese
Agree with the beaten path
I don’t know about the planned kill. It seems that the killer knew something about the family, true.
Sometimes I wonder if the person who came was suicidal and planned to kill himself in front of the family. Mentally, I see him walk in through the front door and putting on slippers because he is Japanese. But…something goes differently, he leaves (very angry), waits, gets into the house via the bathroom window, annihilates the family and then stays because if he is caught in the house, he still has the knife to kill himself. Then the emotional state is gone, and he leaves.
By the way, what happened to the second knife, does anyone know? Was it found in the house, or disappeared? If it was in the house, did he take a new knife with him?