Although Napoleon intended to kill Snowball outright, Snowball's escape turns out to be a great thing for his propaganda mission. With Snowball gone, Napoleon can use him as a bogyman, forever trying to sabotage Napoleon's "altruistic" deeds. The fear of Snowball's mischief is so strong that it overpowers common sense:
Whenever anything went wrong it became usual to attribute it to Snowball. If a window was broken or a drain was blocked up, someone was certain to say that Snowball had come in the night and done it, and when the key of the store-shed was lost, the whole farm was convinced that Snowball had thrown it down the well. Curiously enough, they went on believing this even after the mislaid key was found under a sack of meal.
(Orwell, Animal Farm, msxnet.org)
This all works to Napoleon's advantage, not least because he then uses his dogs to execute animals forced to confess their "collaboration" with Snowball. From that point on, Napoleon is able to use fear of being thought a traitor to control the animals; he can blame every setback -- many of which are due to the failures of his own ideas -- on Snowball, and not only can Snowball not defend himself from the allegations, none of the animals dares to contradict Napoleon for fear of being killed.
napoleon can always blame his errors or anything else that goes wrong on someone and there is really no proof against or for snowball because he is not there. That makes it easy for snowball be blamed. An example is when the windmill breaks down due to the wind and the pigs blame snowball for vandalising it.