Why might it be dangerous for Boxer to say he does not believe Snowball was always a traitor?
This is an interesting concept to think about as it relates to the portrayal of propaganda in the text. Boxer, represents a body that knows the truth. Snowball didn't want to be a traitor, the rest of the animals were made to think he was... and now, as your question displays, the further propaganda that these animals are being fed demonstrates the lie is growing.
If you hear a lie enough times, you begin to believe its true.
For Boxer to be the lone dissenter while everyone else is believing the lie could turn everyone else against him. He must be careful how he proceeds in his steps.
This wouldn't be the case in a democratic society in which everyone has their right to representation and to say what they feel. In their Animal society, they did not have that right.
It is a dangerous move for Boxer to say this because bad things can happen to anyone who opposes Napoleon and the others in the leadership of Animal Farm.
We know that Snowball has been chased off the farm because he disagreed with Napoleon. We also know that Napoleon has the dogs who are willing to do violent things to anyone who Napoleon tells them to attack. In fact, just before Boxer says this (this is Chapter 7) the dogs have killed the hens for rebelling. Therefore, it would be dangerous for Boxer to say anything that might annoy Napoleon.