My theory lies in the fact that the "real" RedJohn is still alive, and that he has been using people as pawns to do his dirty work, killing people.
So the sherrif is indeed "the redjohn" that killed Jane's wife and daughter, but the one who ordered this crime, the real Red John, is still free and alive.

We see here a Red John that gives Patrick what he wants, the person who really commited the crime. Still, why? Maybe he wanted Patrick Janes to commit a crime and feel how RJ feels, so they can become friends? Big question mark!

For my choice, I'm going for Oscar Ardiles.