I feel like Makoto just became more of his true self. As an human, he was stifled and forced to conform to societal standards. He was already a twisted individual and J abandoning him at the brothel forced that darkness out.
I agree with wanting more of J and W's past. I think this story would've really benefited from having 4 volumes instead of 3. We should've seen more of J falling apart during that 100 year skip. J is always so calm and collected. Nothing ever phases him. Seeing J shed that perfect persona and slowly descend into ruin would've made this a perfect story for me.
As for K, I liked them and I'm perfectly fine with how their role was played out.
I just finished reading this and I am not sure how to feel. The premise of the story is indeed fascinating, watching Mako chasing J and playing games with each other was thrilling. Having said, that I felt as if I never truly understood the characters. Starting with M, at fist he was a fuck up but surprisingly gentle boy. After the first time skip he changed so abruptly and I couldn't keep up. He turned into this evil creature extremely fast, which I guess was the whole point but stil I don't get it. His love-hate for J was written very nicely, yet his change of attitude wasn't. He was suddenly a monster. As for J, I get it, loneliness and stuff, I think I just expected more (?), and I definitely wanted to see more about his metor and the extra eye he had. I honestly don't even think I got his past...I also disliked how they handled Ks character. Don't get me wrong the story had some very powerful moments and it is quite special.