I think TS has some strong points. There are some other ideas I've had.

Does anyone feel that there might be an undercurrent of Oedipal Complex in Roland's relationship with his mother?

That maybe seeing her with Walter not only pissed him off but made him jealous also?

Also there's the wanting to save her part. If she hadn't hid, and they had actually had it out, do you believe she would have still tried to escape with the grapefruit? Is it possible that he could have convinced her to stay and confess?>

And, if she had stayed and confessed, would Steven have let her live, and taken her back?>