David Herren appeared in The Blood On His Hands. He is the killer of Julius Coles.
Julius was the killer of Celia Jovanovich. Celia and Julius were lovers, however Visualize had determined that Celia and David were to be soulmates. Celia had decided to come out and be honest with Visualize about her relationship with Julius. Visualize was Julius's only client, so this revelation would have destroyed his career. He killed Celia to keep her silent.
However, as David's soulmate was killed, he decided to kill Julius. Julius had made a deal to cooperate with the FBI on a case against Bret Stiles, thus making David's murder of Julius very suspicious, although David insisted he came up with it all on his own.