One of the best examples concerning the injustice of "IS" is the killing of Alan Henning, a British taxi driver and charity worker who was kidnapped and killed while he was driving a convoy delivering aid to the Muslims in Syria on behalf of a Muslim charity. They did not spare his life even despite pleas for his release by the Muslims, Mujahideen and Ulama from around the world.
Contrast this with the Prophetic example. Mut'im ibn Adi was a mushrik who gave protection to Muhammad (peace be upon him) in Makkah. After the battle of Badr when the Muslims had captured a number of kaafir prisoners of war, the Prophet said of them: "Were Al-Mutim bin Adi alive and interceded with me for these filthy people, I would definitely forgive them for his sake." (Bukhari 4023)
ISIS did not release one British aid worker who was helping the Muslims of Syria, despite countless Muslims appealing for his release.
The Prophet Muhammad would have let the captured kuffar who were at war with the Muslims go free, if one single kaafir had interceded for them.
What a big difference between the two.