Who killed boxer in Animal Farm?

Boxer dies in Chapter Nine when the pigs sell him to a "'Alfred Simmonds, Horse Slaughterer and Glue Boiler,... Dealer in Hides and Bone-Meal." He is injured in his attempt to defend the windmill. However, Squealer weaves a brilliant and moving tail of how no expense was spared by Napoleon on Boxer's medical treatment, and that Boxer's last Words praised Animal Farm in particular and Napoleon especially. This is a brilliant example of the cold and cruel exploitation of the loyal working classes and Propaganda.