Why does Napoleon try to rewrite history in order to further his own ends as well as banish Snowball with no justification in Animal Farm?
This novel presents us with history as a flexible entity, that is open to manipulation and change to suit the ends of Napoleon and his cronies, the other pigs. Especially after the expulsion of Snowball from the farm, we see that history is re-written again and again to maintain control and allow Napoleon to adjust his rule to various and changing circumstances. Note the way in which, almost straight after Snowball leaves, Napoleon suddenly changes his mind concerning the Windmill, but of course, this is not presented this way:
That evening Squealer explained privately to the other animals that Napoleon had never in reality been opposed to the windmill. ON teh contrary, it was he who had advocated it in the beginning, and the plan which snowball had drawn on the floor of the incubator shed had actually been stolen from among Napoleon's papers. The windmill was, in fact, Napoleon's own creation.
The "truth" is constantly re-written throughout the novel, and, as your question says, this is to ensure that Napoleon maintains his own power and does not present himself as weak, vacillating leader that changes his plans and strategy.
To a great extent, the question here is the answer. Napoleon recognizes early on that in order to consolidate his own power, he has to present himself as both a historically credible leader as well as eliminate any potential rival to consolidate his own power. This involves rewriting history to show that he is a ruler that has the backing of the past narrative. In doing so, he also is able to ensure that any lingering support for Snowball is eliminated, making him the only viable option for leadership on the farm. Napoleon recognizes that there is a certain power in being able to present oneself as a historically bound ruler, someone that has the "blessing" of the past to rule in the present and the future. In his recreation of the past with Squealer, Napoleon is able to demonstrate a control of the present and the future with the blessing of the past. In this, he is able to "airbrush" Snowball out of the picture, guaranteeing greater control for his own rule.