"Who is responsible for making the decision to sit Thornton for that last game ?"

 The third and fourth lines were arguably the most consistent all year long, so why fiddle with them?  By switching the lines around and sitting out Thornton
Julien sucked out the little energy the Bruins had left !

Shame on you Julien !  Your job is to win games by playing the best hand you've got at the time.  Everyone with eyes could see that the first line was completely hopeless and should have been playing 4th line minutes .