Over the weekend, the militant organization known as ISIS—or The Islamic State—murdered a British aid worker, posting a video of his beheading online. According to The Daily Beast, the executioner of David Haines, a 44-year-old British citizen who spent the last 15 years as an international humanitarian worker, was the same man who killed James Foley and Steven Sotloff in other recent execution videos posted online. The video reportedly shows Haines reading a prepared statement holding U.K. Prime Minister David Cameron “entirely responsible for [the] execution,” for entering “voluntarily into a coalition with the United States against the Islamic State.” In a statement, Cameron said, “We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes” …