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.