While my opinion is not as "foil bonnet" as Nerds, I think the passing of Irv also had a big part in it. With no accountability between his passing and the time Thompson showed up, Favre was surrounded by enablers. Losing that angered Favre greatly. Nobody tells him no or they will have to pay. Which is why he tanked the '05 season.
When Favre said "don't fire Sherman or I will quit", and they did, he kind of lost it. He pushed Thompson to get Moss, who Favre really didn't care about, because he wanted an issue to walk out over. He actually left Green Bay, went back to Mississippi and told Cook to arrange the deal. Thompson refused to even discuss it. McCarthey had to go talk Favre down and ask him to come back. Favre did, but he still tanked the '06 season.
Having his "buddies" get hired in MN he probably figured he could go back to doing whatever he wanted to in Minnesota. Knowing that Thompson would never let him do that, Favre probably went to Chilly for advice. Between them and Cook, they hatched the escape plan. My reason for including Chilly is that Favre had a "change of attitude" where he worked really hard to have a good season. If he had the third consecutive season with a 70 rating, Chilly probably wouldn't be able to convince the Vikings to pick him up. Providing he could get out of his contract. Because he would appear washed up.
Having Thompson foil his plans by getting the exemption from the league was the last straw. Which is why Favre had to run to the Lions and "other teams" and decode his playbook for them. My guess is that it was Minnesota, because they were admittedly in talks with Favre at the time, and Chicago. Because that is where Favre would do the most damage. Nobody tells Favre what to do or he will make them pay.
Which is why Favre actually played fairly well in '09. He was so mad that he did all the things that the Packers told him to do when he rebelled.
I don't know if Favre threw the pick against the Giants in the NFCCG on purpose. He proved his point, he was cold and disinterested in the game. They were keeping it close in spite of Favre's disinterest. Favre could have just wanted to get it over with. So he floated up a duck to end the game. I don't know. But I wouldn't put it past him.
With a borderline personality disorder, Favre wouldn't need a lot of reason to want punish the Packers, Rodgers and Green Bay in general. Just saying no to him would have amounted to a life threatening insult. He would see sabotaging as an appropriate level of response for having been denied anything he wanted.
Nobody says no to him, that is just unthinkable. So he doesn't have an appropriate response to it. Whether it is the Packers or a married therapist he wanted to nail. He just doesn't understand when someone says no.
That "no" is all that was needed to set him on that path of distruction.
I want to go out like my Grandpa did. Peacefully in his sleep.
Not screaming in terror like his passengers.