Patrick Jane (Simon Baker) confronted and killed a man (Bradley Whitford) who claimed to be his mysterious nemesis in the show's recent third season finale.
"Jane wanted to know if this was Red John or not, and the guy gave him proof that he was," Heller told Entertainment Weekly. "[But] the question remains, was that Red John? The larger question is, how does Jane get away with murder? Whoever the man was, you're not allowed to do that."
Heller refused to confirm whether or not the man that Jane killed was Red John, suggesting that "the viewer is supposed to be convinced".
"Patrick Jane is certain it's Red John [but] some people are not so certain," he explained. "Red John is a master of the mind game. If Red John wanted to die, maybe this is how he wanted to die. Or maybe he just wants Jane to think he's dead."
The showrunner also insisted that the storyline involving the serial killer is not complete, regardless of the dead man's true identity.
"If you... killed your worst enemy, would that be the end of the story?" he teased. "No, it would be the beginning of a whole different story."
The fourth season of The Mentalist will air on Thursdays at 10/9c in the fall on CBS.
Watch Patrick Jane confront Red John on the Mentalist below: