Simon held a grudge against the king and was called upon by Matthew Bonnellus to lead the revolt in the capital city of Palermo in 1161.
This Simon did, with his nephew, Tancred of Lecce, the bastard son of Roger of Apulia, on 9 March.
Simon and Tancred invaded the palace, detained King William, Queen Margaret, and their two sons, and incited a massacre of Muslims.
Originally, the older of the king's sons, Roger IV, Duke of Apulia, was destined to be crowned in place of William, but soon the populace supported the accession of Simon himself.
Instead, the new king's uncle, Henry, Count of Montescaglioso, laid claim to Taranto and all of Simon's former territories.