An SAS sniper was able to stop  at least one execution of hostages by ISIS captors:
A British sniper from the elite SAS saved an 8-year-old boy and his father from an Islamic State executioner by shooting the executioner in the head last month.
The SAS sniper team was reportedly tipped off to the execution in the Syrian desert by an Iraqi spy. When they arrived, they found that several Shia Muslims had already been beheaded by their captors. The IS (commonly referred to as ISIS) executioner, flanked on both sides by armed companions, was preparing to kill a young boy and his father next when the SAS team deployed its .50-caliber silenced sniper rifle.
“The ISIS thug who was about to decapitate the father was shot in the head and collapsed,” an unnamed source told the Daily Star . “Everyone just stared in confusion.”