James isn't a bad person. Or, rather, James isn't inherently a bad person.
What I'm trying to say here is that James didn't kill his wife because he's a bad person; James is a bad person because he killed his wife.
Some people call James evil, but James Sunderland is an ordinary man; there's nothing about him that particularly predisposes him to murder. He could have been any one of us. I think dismissing him as evil means flattening his characterisation to 'he was a terrible person', rather than the more interesting 'he was a desperate person who did a terrible thing'.
Does any of this make James's actions okay? Absolutely not. He murdered a person he loved for selfish reasons; he couldn't recover from his loss while it was still happening, and he convinced himself that his own situation was more important than Mary's remaining life. But that doesn't mean his love wasn't real. If he hadn't loved Mary, maybe he'd have been able to cope with her illness; maybe he wouldn't have longed so desperately and so dangerously for the moment it was all over.
And, perhaps more significantly from our perspective: if he hadn't loved Mary, Silent Hill 2 would be a much less interesting story. James's journey through Silent Hill is meaningful because his love and his regret are real.
I'll never be over the fact that James kisses Mary just before he kills her. Absolutely devastating.
I often have to sit on my hands so I don't end up yelling 'look, I'm not endorsing murder, but how dare you suggest he didn't feel bad about it' at strangers on the Internet. (A lot of the thoughts on this page were provoked by someone saying that James 'effectively doesn't show any remorse over his actions because he doesn't feel like he did anything wrong'.) It's rough when your favourite character literally murdered his wife and you're left going 'I'M NOT SAYING WIFE MURDER IS OKAY, BUT'; it's just never a good opening line.
Silent Hill 2 isn't a story about a monster; it's a tragedy. In another world, perhaps James could have been happy; perhaps he could have deserved to be happy. But chance and weakness led us to this point, and now there's no going back.