What Gulliver says that causes the Emperor to turn against him is that he would never help bring free and brave people into slavery.
In "Gulliver's Travels," written by Jonathan Swift, the protagonist is reluctant to fulfil Lilliputian emperor's desire of destroying the Blefuscudians. The emperor aims to become the sole monarch in the entire world, however, Gulliver asserts that he "would never be an instrument of bringing a free and brave people into slavery." Consequently, he is accused of treason.