Factual error: Myra's pet "chameleon" is some other type of lizard, not a chameleon.
Continuity mistake: After stealing the car, you can see Mr. Goames amend the Almanac for auto theft from $226,530 to $226,531, but in future shots, no amendment is visible.
Answer: I don't think Ben always planned to murder his wife. He wanted to "have it all": stay married and carry on a relationship with Sharon (or someone else - he may have done this before). He lied to Sharon about his wife refusing to grant him a divorce. As Sharon's mother discovers, his wife knew nothing about the affair. She believed that she and Ben were happily married. Ben decided to kill her later, after she heard about Sharon (from Sharon's mother). She pointed out that he would not receive much money if they divorced. Presumably, he thought he could inherit money if she died.