According to encyclopedia.com, following on the Protestant Reformation,
In 1580, outraged at the deaths of Catholic missionaries, Pope Gregory XIII made a pronouncement that encouraged Elizabeth's murder, saying, as quoted in translation by Alison Weir in The Life of Elizabeth I, it would be justified to kill: "that guilty woman who is the cause of so much intriguing to the Catholic faith and loss of so many million souls. There is no doubt that whoever sends her out of the world with the pious intention of doing God service, not only does not sin but gains merit."
In the event, Elizabeth I was not assassinated; but were there other occasions when a Pope sanctioned the faithful to kill someone? If so, did it happen?