Ray Allen ultimately chose playing time over loyalty and no one, Celtics fans included, should blame him.
Allen was the third wheel in the Big 3 marriage from the start, as he had the most difficult time adjusting from being THE MAN in Seattle for the Supersonics to the third wheel (and more recently fourth or fifth wheel) in Boston.
Miami simply wanted Allen more. The Celtics, who had tried to ship Allen out of town on multiple occasions, hadn't REALLY wanted the NBA's all-time 3-point shooting leader for some time now. In the past week, the Heat wooed Allen like no one since Jim Calhoun did way back in the early '90s when Allen was a wide-eyed teenager. Miami promised him the world, except more money.
Whenever an athlete takes less cash, he is typically applauded. Not Allen.
No. 20 will be booed everytime he touches the ball in Boston as the perception is that he joined the NBA's evil empire. Of course, rational minds shouldn't boo him. Allen is arguably the classiest player in the league.
The Celtics? They'll be fine. Young Avery Bradley will get the playing time he deserves. New sixth man Jason Terry (not a bad shooter in his own right) will get the playing time he deserves. One could easily make the case that even without Allen, Boston is the deepest team in the league (saying that it re-signs Jeff Green).
Don't expect Allen to badmouth Boston. Don't expect him to leak anything (Red Sox-style) negative about Rajon Rondo.
Ray Allen will forever be the epitome of class. He just no longer will be forever thought of as a Celtic. Just a mercenary. And, from the beginning, that's all he ever was anyway.