why does Snowball plan to murder Napoleon?

Napoleon did not live up to his promises, and Snowball had been driven away. Snowball's plot is confessed by the three hens that are then executed...... was it a true plot? We're never told.

"At the same time there were renewed rumours that Frederick and his men were plotting to attack Animal Farm and to destroy the windmill, the building of which had aroused furious jealousy in him. Snowball was known to be still skulking on Pinchfield Farm. In the middle of the summer the animals were alarmed to hear that three hens had come forward and confessed that, inspired by Snowball, they had entered into a plot to murder Napoleon. They were executed immediately, and fresh precautions for Napoleon’s safety were taken. Four dogs guarded his bed at night, one at each corner, and a young pig named Pinkeye was given the task of tasting all his food before he ate it, lest it should be poisoned."