Here are some of the major incidents where the Islamic State killed the hostages.
Link to full list from Washington Post here. (pictures are non graphic)
FLOD: The Washington Post has documented some of the more notorious acts of violence carried out by ISIS over the last year. It would be impossible to include all the murderous acts of barbarism, since that would range in the tens of thousands and most have gone undocumented. If ever this page posted a warning sign for the world to take notice, it would be the cruel, violent advancement of ISIS.