Learning From Mistakes
He learned from his mistakes so we doubt that he will be in the same spot again. But, after looking at the way she was acting, he was in complete shock back then. The strange behavior she displayed made him wonder if she was faithful to him.
Opportunities
After all, since he was always away, she had plenty of opportunities to have an affair. What made it worse was that she used his money for her affair. It would be a double whammy of betrayal if this was the case. He was a clever guy, so he had a plan.