He must have been accused of more than just theft, the penalty for which in Sharia is amputation of the hand (Qur’an 5:38). Or perhaps the scale of his thefts and embezzlement made it eligible to be considered as spreading corruption in the land — and one of the Qur’anic penalties for that is crucifixion.
“Indeed, the penalty for those who wage war against Allah and His Messenger and strive upon earth [to cause] corruption is none but that they be killed or crucified or that their hands and feet be cut off from opposite sides or that they be exiled from the land. That is for them a disgrace in this world; and for them in the Hereafter is a great punishment” (Qur’an 5:33)
“Islamic State militants behead senior member for theft: Monitor,” AFP, November 15, 2014 (thanks to Twostellas):
BEIRUT (AFP) – The Islamic State (IS) extremist group has executed a senior member who was accused of embezzling funds and theft, the Syrian Observatory for Human Rights monitor said on Friday.
“The Islamic State executed a senior leader in the group of Syrian nationality, beheading him and hanging his body on a cross” in eastern Deir Ezzor province, it said.
The man was accused of “taking money from Muslims… and embezzling Islamic State funds”, the Britain-based monitor added.