It all makes total sense! He is just like Jane. He is charming, manipulative, and arrogant. Mashburn loved being a murder suspect in that past episode. He insinuated himself into that investigation and played with Jane like a cat plays with a mouse. He is intelligent. He is abhorrently wealthy which gives him unbelievable power and resources. His past romantic relationships were superficial and typical of men with narcissistic personality disorder. He no-doubt considers his conquer of Theresa Lisbon to be another victory in his pocket.
I know that the episodes with Mashburn were played very light hearted and comically but I also think that his character could very easily turn evil. I just haven't been able to think of a reason WHY Mashburn would be Red John. Not enough history, I guess.
This also gives CBS a GREAT way to amp up the romantic angst for all the Jane/Lisbon shippers out there. If Walter returns he will obviously try to seduce Lisbon and his vendetta for Jane will escalate to a murdering point. Enter "Lisbon-In-Peril" scenario and Jane will have to outsmart a man who is literally as smart as he is.
