I don't think Kristina is Red John. I have many reasons but the most logical one would be she is now too obvious! And when creators make someone obvious, it doesn't usually end up correct!
Though I would not rule out her as an accomplice of Red John! I mean, her indifference when Jane mentioned that she made a mistake, claiming that she said the truth about Red John and things like that.
The twist in guessing who Red John is, is that in those episodes, when they showed that shadow and tea cup (1x23) and the man who poisoned Rebbecca and even when Rebbecca and Rosalind claim that they loved Red John, in the show they never mention that is Red John himself! They could be accomplices for all we know!