Even Boxer does not believe that Snowball was always a traitor. Why is his saying a dangerous move? What clues are given in the text?
Boxer was the largest and most powerful animal on the farm. He had earned the respect and admiration of every animal because of his dogged determination to do his best. He worked twice as hard as any animal and was extremely loyal. In addition, Boxer lacked intelligence, which meant that he could easily be brainwashed into believing almost anything. He had adopted two maxims, 'I will work harder' and 'Napoleon is always right,' which epitomized his dedication and allegiance to the cause.
Napoleon and the other pigs obviously took great comfort in the fact that Boxer was so obedient. Greater security was also found in the fact that he was so stupid. Both these factors created the assumption that Boxer would never be a threat. Since he was so much respected and so strong, it would have been easy for him to take the lead in a protest against whatever abuses by the pigs he might have identified. The other animals would have been more than willing to support him.
Considering these factors, it was, therefore, quite a shock to Squealer when Boxer, the docile and faithful giant, questioned his statement about Snowball being a traitor since he had always had an unquestioning belief in whatever he was told. He had, for example, after Snowball's expulsion, easily accepted the declaration that Snowball was a criminal by stating that, “If Comrade Napoleon says it, it must be right.” Squealer, after Boxer's opposition, must have become acutely aware of the possible danger a rebellious Boxer could hold. However, when he stated that Comrade Napoleon had categorically stated that Snowball had been Jones' agent from the very beginning, Boxer acquiesced and stated:
“If Comrade Napoleon says it, it must be right.”
Boxer's acceptance obviously did not alter Squealer's suspicion that he presented a threat, for "...he cast a very ugly look at Boxer with his little twinkling eyes. He turned to go, then paused and added impressively: “I warn every animal on this farm to keep his eyes very wide open. For we have reason to think that some of Snowball’s secret agents are lurking among us at this moment!” He was clearly alluding to Boxer in this instance.
Because he had initially been argumentative and in opposition to what Squealer had said, Boxer had unwittingly put himself at risk. He later did much the same when, in chapter eight, he questioned Squealer about the attack on the windmill by Frederick and his men and whether the animals' success could be deemed a victory or not since the windmill had, in fact, been blown apart.
It was therefore no surprise when, four days later, three of Napoleon's dogs attacked Boxer, who easily overcame them by putting out his massive hoof and pinning one of them to the ground, at which the other two fled. The dog shrieked for mercy and Boxer only released it on Napoleon's terse command. This is an obvious indication that Boxer would, from then on, be regarded with greater reservation and that he would probably be removed at the first opportunity.
Unfortunately for Boxer, this opportunity presented itself when he fell desperately ill in chapter nine. The pigs called on the knacker to remove him. They later explained that the knacker's cart was actually that of the veterinarian who had not yet removed the knacker's signage from the vehicle after purchasing it from him—a story the animals generally believed. However, Boxer apparently later died of his illness. Soon after, the pigs bought a case of whisky with money they found somewhere. Clearly, Boxer had been sold to the knacker and the malicious pigs were remorselessly enjoying the profits from their sale.
In George Orwell's Animal Farm, Squealer tells the animals of Snowball's alleged treachery, Boxer has a difficult time accepting it. "I do not believe that" (31). Squealer has to redirect Boxer three times and Boxer is only convinced when Squealer tells him that "Our Leader, Comrade Napoleon . . . has stated categorically . . . that Snowball was Jones's agent from the very beginning" (32).
It is only then that Boxer accepts Squealer's accusations of Snowball. Apparently Boxer's persistent defence of Snowball's innocence was too much of a challenge for Squealer because, as he left, "it was noticed he cast a very ugly look at Boxer with his little twinkling eyes" (32).
Four days after, four pigs were executed. A few of the dogs, who appeared to go mad after they tasted blood, jumped at Boxer who prepared to kill one of the dogs until Napoleon ordered him to release it.
It's clear that Boxer was perceived as a danger by Squealer and Napoleon and the failed attempt at execution/assassination only put off the inevitable. Boxer was marked for death from that moment on and when his strength finally gave way, Napoleon took the first opportunity to make a profit and rid himself of the problem.