we put a stationary team around Pirlo for his last few years here, which limited his effectiveness. He always put in a lot of running and was usually covered the most ground of any of our players. When the team got old the mobility decreased which led to a decline in his performance, it wasnt really that his effort was all that different.
When we decided we didnt want him anymore he decided he wanted to stay in Italy, and that left Juve as the logical choice, considering that they are probably the only other team in Italy (besides for us and Inter) that could afford the wages that he wanted, and you cant blame him for wanting to go to a team where he felt he could win a title.
Once he went to Juve, he got the mobile team around him that he had not had here for some time, and after he had those players around him he became much more effective again. I dont see how that makes him a traitor.