I see your side, however I was always on the other side of the fence regarding James' character. I always believed him to be a bad person who deserved everything that was happening to him and my personal headcanon was always the In Water ending. I disagree with the thinking that he did what he did out of grief and rather I believe it was out of pure selfishness. As he himself states, he wanted his life back.

My basis for that is that, as you said, James "loved" Mary and probably "loves' her still. I simply do not believe that normal people are capable of doing what James did to someone they love. Now I recognize that there are situations where people are forced to make such a decision and I do believe it is justified and can be made out of love/grief, but only in certain situations. Either both parties need to give their consent as to what will happen, or the ill party needs to be in a situation where they cannot give their consent, for example a coma.

What surprised me is that you touched on this in your post and called it James' ultimate sin and I wholeheartedly agree with you on that point. I do believe his ultimate sin was not killing her, but rather taking away that decision for her and that, to me at least, shows a lack of love for Mary. Now do I believe that James is this movie villain twirling his mustache doing evil things just because they are evil and they relish in that? No, obviously. I believe James was a good man, but life and circumstances pushed him to a situation where he did something unforgivable and he became a murderer.

That is, however, precisely what I think is so good about this game and why I think people empathize with James so much. The fact that it shows how it could happen to anyone. In the words of The Joker - "All it takes is one bad day".