1) Orwell uses Boxer’s death as a searing indictment of such totalitarian rule, and his death points sadly and bitterly to the downfall of Animal Farm. The great horse seems to have no bad qualities apart from his limited intellect, but, in the end, he falls victim to his own virtues—loyalty and the willingness to work. Thus, Boxer’s great mistake lies in his conflation of the ideal of Animal Farm with the character of Napoleon: never thinking for himself about how the society should best realize its founding ideals, Boxer simply follows Napoleon’s orders blindly, naïvely assuming that the pigs have the farm’s best interest at heart. It is sadly ironic that the system that he so loyally serves ultimately betrays him: he works for the good of all but is sold for the good of the few.
http://www.sparknotes.com/lit/animalfarm/section9.rhtml
As far as Benjam in being partially responsible, I'm not sire he is. He stayed with Boxer while the other animals went for help, and he was the one who read the sign on the truck and told everyone that they were taking Boxer to the glue factory, but I wouldn't say that had he done things differently he woyuld have been able to stop Boxer's death. Just an opinion though.....