Charles de Louviers, Lord of Maurevert, is considered by all of his contemporaries as the assassin who targeted – and missed – Admiral Coligny, leader of the French Protestants, on 22 August 1572. This act opened the way for a Parisian political crisis which culminated in the St. Bartholomew’s Day Massacre, on August 24. This assassin, curiously little-studied, merits a financial investigation: a professional killer must be paid, and the resulting financial activities and other forms of compensation allow us to better determine the identity of his sponsors. Thanks to a notarized contract between Maurevert and the Duke of Guise, we can confirm that the latter eminent figure was his first sponsor and main protector. Furthermore, details within the contract, linked with other types of compensation in money and honours, clearly indicate that Guise did not act alone: powerful royals, notably Catherine de Medici and the future Henry III, were among the organizers of the attempt on Coligny’s life. Finally, this article shows that one of the main authors and historians of the era, Jacques-Auguste de Thou, had family ties with the murderer and one of his probable accomplices, Georges Postel, Lord of Ormoys.