Today is Beheading Day in Hatred and Violence in the Qur'an Awareness Month:
[Remember] when your Lord inspired to the angels, "I am with you, so strengthen those who have believed. I will cast terror into the hearts of those who disbelieved, so strike [them] upon the necks and strike from them every fingertip." (Qur'an 8:12)
So when you meet those who disbelieve [in battle], strike [their] necks until, when you have inflicted slaughter upon them, then secure their bonds, and either [confer] favor afterwards or ransom [them] until the war lays down its burdens. That [is the command]. And if Allah had willed, He could have taken vengeance upon them [Himself], but [He ordered armed struggle] to test some of you by means of others. And those who are killed in the cause of Allah - never will He waste their deeds. (Qur'an 47:4)
These passages are the chief justifications for the Islamic practice of beheading hostages and war captives. At the beheading of American hostage Nicholas Berg in May 2004, for example, the late Iraqi jihad leader Abu Musab al-Zarqawi invoked the Battle of Badr, a discussion of which is the Qur'anic context of 8:12. "Is it not time for you," al-Zarqawi asked Muslims, "to take the path of jihad and carry the sword of the Prophet of prophets?…The Prophet, the most merciful, ordered [his army] to strike the necks of some prisoners in [the battle of] Badr and to kill them….And he set a good example for us."
One that many still follow.