In a sense, Boxer's betrayal is the true climax of the slow loss of liberty and freedom that the animals experience; this moment sums up all previous events. Napoleon never intended for old animals to be retired; if they are being fed without doing any work, they are a detriment to the farm and have no purpose. Boxer has literally given his health to the farm, working harder than any other animal and believing fervently in Napoleon as a leader, and his reward is to be sold to a glue-maker:
...there was the sound of a tremendous drumming of hoofs inside the van. He was trying to kick his way out. The time had been when a few kicks from Boxer's hoofs would have smashed the van to matchwood. But alas! his strength had left him...
(Orwell, Animal Farm, msxnet.org)
As demonstrated by real-life events in Russia, this is the reward for serving a dictatorial regime; the harder they work, the more work they get, until they fall and are ground under the heels of new workers. The regime can only prosper -- for itself, not for its workers -- when the workers are kept believing that there will eventually be a reward, or that their work will benefit their children. This moment in the novel cements Napoleon as a ruthless dictator instead of a beneficial leader, and shows how the ruling elites not only do not appreciate the working class, but actively treat them with contempt.
Betrayl has been an overriding theme in the works of several writters in the past. in this text, the betrayal of Boxer can be viewed as an alternative climax in the text for the fact that, all the animals collectively staged the rebellion, boxer worked hard for it to work out, but at the long run, all his efforts we relegated to the background, especially when he was discovered to have overworked himself. At this juncture, it becomes obvious where the text is heading towards and the fact is made clear that, the original aim of the rebellion has been compromised and thensupense level of the reader is hieghtened.