Joe explained to Beck how he visited Mr. Mooney after he and Candace broke up. He found Mooney lying on the floor after suffering from a stroke two days prior, which left him paralysed and unable to speak.
Joe blames himself because he should have been there. He says he’s made an effort to visit Mooney every week since, so it does sound like it could be one of Joe’s genuine, regrettable mistakes. However, this fan theory puts forward the possibility that it was Joe who caused Mooney’s stroke.
There are a few reasons why Joe could want to hurt Mooney. The first is that he knew he killed Elijah, the man Candace had an affair with. The second is that he was getting revenge for the abuse he suffered at Mooney’s hands when he was younger.
Despite this, Mooney is the only family Joe has ever really known, which might explain why Joe paralysed him instead of killed him. On the other hand, maybe he really did try to kill Mooney but something went wrong.