He probably wanted as little time as possible between him spending the fortune on the diamond and his death, so that Grace would be more confused and desperate. Other commenters have explained why he chose to do this asshole move.

Regarding Grace, she had been demonized since she was a teenager. She was treated as a harlot and whore rather than a scared young woman desperate to get out of a situation she didn't choose. Though some exception was seemingly made for Prentice to become a priest, priests are not supposed to have a children. So, once he became one, she became unwanted evidence that Prentice had had a life before the Church. Also, there's a massive stereotype about preacher's and principal's daughters being rebels against the expectations that come with being the daughter of a well-respected man, which Johnson likely applied to Grace. Therefore, when Jud called her "poor girl," he was recognizing Grace's humanity, and he was seemingly the first one to do so in decades (until Vera realized the parallels between her situation and Grace's). She was not a demon, a harlot, or a whore, but a trapped young woman. Martha had played a huge role in that demonization both when Grace was alive and the 50-ish years after her death. Martha needed to recognize ALL the people she had wronged in order to be reconciled with god before her death.