SOLVED. Japanese YA literature [s]

[Ben]

A teenage boy spies on mother through a peephole in his closet. Convinces his friends to plot the murder of a sailor who is seeing his mother. They decide they have to do the murder as teens so they haven't adult legal responsibility.

[Lobstergirl, au gratin]

What year did you read it? Did you read it in Japanese or English? Author male or female? Remember the cover? Is it set in Japan, with a Japanese protagonist?

[Ben]

It is set in Japan. All Japanese characters. Written the sixties. Not sure about the author except quite famous in Japan. English translation. Plain cover.

[Tab]

Sounds like The Sailor Who Fell from Grace with the Sea

[Ben]

That's it! Thank you!