The persecution of French Protestants was begun by Francis I and increased under his son, Henry II.
Henry’s boyhood friends, the Guise brothers influenced him to take more drastic action against the Protestants.
Their sister (Mary of Guise) was married to King James of Scotland. Their niece (Mary Queen of Scots) was being raised in the French court. Henry had married his minor son, Francis II to their minor niece Mary Queen of Scots.
They saw themselves as power brokers to keep Catholicism in both France and England. They encouraged Henry’s hatred for Protestants (called ‘heretics’ by Francis’ Edict of Toleration in 1540) and participated in executing more extreme measures against the Protestants (Huguenots).
When Henry II died, their machinations precipitated the French Wars of Religion.