Clint Eastwood's Unforgiven does contain certain themes taken from Protestant Christianity; without an understanding of that, it may be difficult to understand William Munny's character, his motivations and reactions.
What is the theology behind the Japanese remake? Perhaps Ken Watanabe's samurai character becomes a Buddhist and decides to 'atone' for his sins; but he doesn't become a monk, which is usually how this is depicted. The Japanese story seems like it is about a "retired" samurai or a ronin who must decide whether to give way to the new industrial age or if he will fight to defend others or for the sake of honor?
Second Teaser For Japanese Remake Of Eastwood's UNFORGIVEN
While doing a search, I found this post by Peter T. Chattaway at Patheos: Unforgiven, samurai remakes, and subverted Christ figures.
The Gospel of Violent Zeal in Clint Eastwood's Unforgiven
"It's a Hell of a Thing to Kill a Man": Western Manhood in Clint Eastwood's Unforgiven