He tried, through appointing his scribe Toto as advocate, to control the monastery of San Massimo and her property, but failed. Together with his wife Sichelgaita, he founded S. Maria de Domno and put it directly under the authority of the archdiocese of Salerno. Under its first abbot, Radoald, it was very successful, though John's attempts to control religion in his principality were less so. In January 984, John associated his son Guido with him, but Guido died in 988. Between January and March 989, he associated his next son Guaimar, who succeeded him. He left other sons in Pandulf, Lambert, John, and Peter.
According to a legend related by Peter Damian, there was an eruption of Mount Vesuvius and John exclaimed that surely it was an omen foretelling the death of some rich man, who would surely end up in hell. The next day, John was found dead in the arms of a prostitute. Perhaps this is the basis for his epithet "accursed".