For Money
As you may know and what is obvious here is the fat Smith did all of that for money. As it turns out, they had cash all over the house. They were planning to steal over $50 million and they ran away to some island where they would be safe.
Obviously, Mary would go with them and they would all live there safely, as much as possible. Sadly, they forget one thing. Sam has been looking for this bank robber his whole life and he even started his Private Investigation company in order to find him. The last thing.