That is a good question, why? The only noble reason would be it was her dying wish - but he only came up with that after the money. Why not let her parents care for her and move on? Why you ask? He wanted her gone obviously. Why do a few husbands kill their wives?
Money. Another woman. Freedom. The things Michael already had without killing her.
"The only noble reason would be it was her dying wish"
Bingo! Hard to believe, huh?