I thought, for a while, that he had an affair with Kelly which ended his marriage. However, when he was talking to Dylan about his marriage being over Dylan asked him if he had cheated on her and he said that he would never have done that. Dylan said that he didn't think he would and ask if the wife had an cheated and he again said no to that too. So that apparently was a wrong guess.
They do seem to be closely connected though. Maybe the brother/sister speculation some have is correct. He seems like such a nice guy the secret and murder thing is a mystery. If he did hit her child, it doesn't explain how it would end his marriage, unless it was DUI or something like that. It is a puzzle. I will be glad when they reveal.