There is no money left.
Isn't there an insurance policy on her life? I'm pretty sure I heard a discussion of that.
In any case, Michael needs her out of the way so he can marry his mistress. Maybe he'll beat her up like he did an ex-girlfriend.
How can you be so sure? Do you know for a fact that there is no 'death' insurance policy that he will get, as long as he stays 'married' to her?