Various sources have received independent confirmation from inside Syria that these men were former Muslims that had converted to Christianity. They were not ‘..rebel fighters from a rival faction..’ as many news outlets are reporting.
Each of the 8 men were charged with the crime of apostasy for renouncing Islam and converting to Christianity. When confronted by the ISIS members, they refused to “revert” to Islam.
This is the “only” reason they were crucified. According to Islamic jurisprudence, crucifixion is a method of death reserved for Christians. They do not crucify other Muslims.