Islamic law forbids Muslims to leave Islam. These murders were revenge upon one Muslim who dared to exercise his freedom of conscience. "NEWS ALERT: Nigeria Militants Kill Children Of Christian Convert, Missionaries Say," by Stefan J. Bos for BosNewsLife, November 23 (thanks to Mackie):

ABUJA, NIGERIA (BosNewsLife)-- A militant group seeking to enforce Sharia, or Islamic law, throughout Nigeria, has shot and killed two children of an ex-terrorist and "murderer" because he converted to Christianity, well-informed missionaries told BosNewsLife Wednesday, November 23.
Boko Haram, meaning “Western education is a sin”, carried out the killings this month after discovering that a former fellow fighter refused to kill a Christian and instead accepted Jesus Christ as his Lord and Savior, explained Rae Burnett, Africa Director of the U.S. based Christian Aid Mission (CAM) group.

Burnett told BosNewsLife that... http://www.jihadwatch.org/2011/11/nigeria-muslims-murder-children-of-con...