Kidnapping infidels and releasing them for ransom or killing them -- whichever is more advantageous for the Muslims -- is fully sanctioned in Islamic law: "As for the captives, the amir [ruler] has the choice of taking the most beneficial action of four possibilities: the first to put them to death by cutting their necks; the second, to enslave them and apply the laws of slavery regarding their sale and manumission; the third, to ransom them in exchange for goods or prisoners; and fourth, to show favor to them and pardon them. Allah, may he be exalted, says, 'When you encounter those [infidels] who deny [the Truth=Islam] then strike [their] necks' (Qur'an sura 47, verse 4)" — Abu’l-Hasan al-Mawardi, al-Ahkam as-Sultaniyyah (The Laws of Islamic Governance), trans. by Dr. Asadullah Yate, (London), Ta-Ha Publishers Ltd., 1996, p. 192.
"Nigerian Islamic extremists claim they killed 7 foreign hostages," from the Associated Press, March 9 (thanks to Twostellas):
JOHANNESBURG — An Islamic extremist group says that it killed seven foreigners kidnapped in northern Nigeria.
The group, Ansaru, said on its website late Friday that it killed the seven Christian foreigners because it said Nigerian and British forces had killed some Muslims while trying to free the hostages.
Nigerian military spokesman Col. Mohammed Yerima told the Associated Press he had no information about any Nigerian/British military operation.
The seven foreign workers — three Lebanese citizens and one each from Britain, Greece, Italy and the Philippines — are all employees of Setraco, a Lebanese construction company, and were kidnapped in mid-February.
Ansaru is a splinter group from Nigeria’s Islamic extremist network, Boko Haram. Boko Haram is currently holding a French family of seven who were captured in neighboring Cameroon in late February.