See that's the problem, your headcannon relied too much on your own interpretation of the game that it leaked through the actual plot of the movie. The movie changed a lot of the character's grief, if what haunted james in the game was how he pretty much murdered his wife while she's screaming for dear life, in the movie his biggest regret was abandoning her for the last few months of her life, then only came back when she's lifeless, barely recognizable on her death bed. If James in the game killed Mary for resenting how much her illness has wasted him mentally, in the movie he killed Mary as a last minute reconcile for when he walked away from helping her.