* Boxer was taken away by the van. The dogs were involved in this, but it is never explicitly stated that they physically attacked him. They were used to intimidate and control the other animals, and they were the enforcers of the pigs' authority.
* The pigs sold Boxer to the slaughterhouse. This is the real tragedy of Boxer's fate. He was a loyal and hard-working animal who gave his all to the farm. The pigs, however, saw him as a burden and a liability, so they profited from his death by selling him to the slaughterhouse.
It's important to distinguish between the dogs' role in the events and the pigs' overall cruelty and manipulation. While the dogs represent the pigs' brute force and control, the real villain in Boxer's story are the pigs who ultimately betrayed and exploited him.