Red John is not left-handed. A left-handed person would not paint with his right hand as Red John did on Lisbon's face.

By the way, where did all the blood on Red John's knife come from? Partridge was already dead and his blood almost dried, so the blood on Red John's knife should also have been dried or dropped off while he was waiting for Lisbon to find Partridge. Did he then stuff his knife in Partridge again to get some blood for his painting? Could he not just put his fingers into Patridge's wounds to pick up blood? And how the heck did he call, talk and hang up the phone while his both hands where busy? Since no reasonable explanations yet has been given about all this, perhaps it's just another blunder by the writers!