What Katie did wrong.
First off, it's likely that mealy mouth sob killed poor Katie. And she certainly didn't deserve that. But I was surprised at a couple of things she did that, had she chosen a different course, it may not have led to the awful result that happened.
First..she stayed after he beat her up. And that wasn't the first time he'd attacked her. She even knew by then he wasn't a nice guy who had just flown off the handle. She definitely knew he had a dark violent side. Yet, she stayed.
Second...she stayed to try to get money out of him. There was a point when she could have left w/o any money. Staying with an abuser out of fear is bad enough, but to stay to try to get money instead of walking away, isn't the smartest thing in the world. I know it'd be hard to do, and she'd have a problem paying for school. But staying to endure being beaten up so you can finish with school? Like he was going to pay for her school then let her walk free to have a wonderful life as a doctor and make a lot of money on her own? No way. He'd confirmed that she married him for money, and now was staying just so he'd pay for her school, after which she'd leave him, after using him.
Third...she tried to get revenge by mailing in the books. And then she mailed them to the wrong person. Naive for a smart woman going to med. school. Her situation was dire enough that what difference did the illegal activities matter, in teh scheme of things? It was just for revenge, and they were too powerful a family for her to get revenge. Bad move. It's possible he killed her to show his family he was not weak and that he'd handled the situation. (remember that when his brother is telling David they're moving his office, after the book mailing incident with the wife, etc., and the father remarks to David that he's weak.)