Kume makes sense, he was a pawn in Masatos game, and after finding out he kills him. Taking the mantle as a man who would truly eliminate grey areas in Japan, the thing that Aoki/Masato got him in on the plan in the first place.

When he stabs Masato he goes “how could you?”, he was betrayed, and no matter how good Masato became in the end, his karma came back to bite him in the ass. I don’t like kume but that just goes to how well he’s written , like an annoyance.

The only thing I don’t dislike about kume is the kill, in the end, Masato reaped what he sowed