He is saying that Updyke is a good person in general, but his obsession with Alabama football has led him to create an alternate persona- "Al from Dadeville", and that obsession is what caused him to kill the trees at Toomer's Corner. Al from Dadeville is the "bad side" of Updyke, an otherwise "good" person.
I'm not sure that there is a diagnosis for someone that is obsessed to the point where they spend 18 hours a day doing something to the point that it affects their life adversely. If there is, it's certainly not bipolar disorder. But, there is without a doubt something psychologically wrong with Updyke.
What he did would be like me as a Cardinals fan killing the ivy at Wrigley Field and expecting Cardinals nation to glorify me for doing it. He is almost a vigilante in his mind- he can't understand why his Alabama brethern have disowned him- talking about how "hurt he was that Nick Saban sent money to help Auburn out with the trees."
Maybe if he researched the consequences for killing those trees, he wouldn't have done it.