Ellsbury is not a traitor or even a "trader". The Yankees are a great fit for him. The Yankees had the money, the years and play in a ballpark that fits his swing.
We won't know exactly what the Red Sox were offering but I am guessing it wasn't close to the Yankees offer. It wouldn't make sense for the Sox to sign Ellsbury to this deal. The goal of a well run organization is to build a team from the farm system up. This is not always possible of course but in this situation it was for the Red Sox.
Jackie Bradley is the future for the Red Sox just like Evan Gattis is for the Braves. When you have the young replacement in place, and under team control for the league minimum salary, you don't re-sign the 30 year old veteran that is blocking him to a 7 year deal for over $150 million. You save that money to re-sign players and sign players in areas where your farm system is not producing. The Red Sox were smart to let Ellsbury walk just like the Braves were smart to let Brian McCann walk.
Xander Bogaerts and Jackie Bradley are important because they can be impact players at a very low salary. This enables the Sox to use their resources to improve the team in other areas. You don't spend money on getting a third car when you need to fix the roof on your house. It was a no-brainer for the Red Sox and Ellsbury. Nobody needs to be mad at either of them.
Ellsbury is a very good player but he is not a superstar. The problem is he is being paid superstar money. This could cause an issue with what Yankee fans expect of him. Could he become the 2011 Ellsbury again? Maybe, but not likely. Ellsbury and the Red Sox are the winners in this deal.