Because he's unfaithful to her. He isn't a husband to her anymore, and he's found someone else that he is apparently fairly committed to.
I don't get why he would stand in the way of someone who wanted to take responsibility for her. I find this insistance on making sure she dies kind of odd given the circumstances, don't you?
Its been 15 years. He feels he is doing what she would have wanted. You have Catholic Priests who take vows of celibacy who can't even keep them. He is just as human as you or I.
i find it one of the driving forces on the side of keeping her alive. conflicted to the max and equally resolute in not giving her up to the care of her ready willing and able parents. reasonable people cannot comprehend it.