I suppose that I would argue that this is because Old Major put ideas into the heads of the animals. These ideas were much more dangerous than any mere actions could be.
If you think about it, ideas are a very powerful thing. Christianity is an idea. So is democracy. Both have taken over large parts of the Western world and had a huge impact on human history.
In this same way, Old Major's ideas could potentially be more dangerous than anything Napoleon could do. His ideas could spread to other farms or could crop up again on Manor Farm -- ideas cannot be killed.