Consulting the original assignment and assessing if there are any stipulations or requirements would be considerably helpful in your task. You might have to include some elements that would help you construct your thesis statement. Outside of that, I would say that your thesis statement might want to revolve around the idea of power. Napoleon holds power over Boxer. He is able to exert this power in obtaining whatever he can from Boxer, who is a complete apologist and supporter of the power structure. When Napoleon no longer needs Boxer, he arranges for him to be killed off. Essentially, when the power structure has extracted what it needs, it discards those who support it and prop it up. I think that a thesis statement around power and hierarchical structures tend to favor those at the top of it at the costs of those underneath could be a starting point. Another avenue that your thesis statement could go would be to the idea of how Boxer refuses to question authority, inevitably causing his destruction. The idea of "I shall work harder," ends up being Boxer's own death warrant. I think that a thesis statement about how loyalty in a political system tends to disempower those at the bottom of the order and empowers those at the top of the structure might be another venue to which the thesis can be directed.