there's no question that on the scale of who deserves pity mary would be near the angela scale of blameless and james would be on the MURDERED HIS WIFE scale of fucked upness. i'm not denying that at all, but even mary thinks that she fucked things up between them and says so in her letter. sure there are people dying of terminal cancer who lash out at their loved ones. there are also people dying of terminal cancer who don't verbally abuse their spouse every time they visit and allow them to support them. as abuse goes it's pretty mild and james being the unreliable narrator that he is i'm not even sure how long it had been occurring but apparently it's enough to severely damage their relationship and cause him to snap.

obviously you shouldn't kill your wife just for yelling at you and he's a damn murdering scumbag for doing so but mary shouldn't have pushed him away and let him be a supportive husband like he wanted. it's a minor fuckup but still a fuckup that she herself realized in her letter, sadly after james commits the obviously more massive fuckup of killing her.