The Islamic State considers itself to be the caliphate, the sole legitimate government to which Muslims owe allegiance. So wanting to leave it is tantamount to opposing Allah and Muhammad.
“There has already been for you an excellent pattern in Abraham and those with him, when they said to their people, ‘Indeed, we are disassociated from you and from whatever you worship other than Allah. We have denied you, and there has appeared between us and you animosity and hatred forever until you believe in Allah alone…’” (Qur’an 60:4)
“You will not find a people who believe in Allah and the Last Day having affection for those who oppose Allah and His Messenger, even if they were their fathers or their sons or their brothers or their kindred.” (Qur’an 58:22)
“O you who have believed, do not take your fathers or your brothers as allies if they have preferred disbelief over belief. And whoever does so among you – then it is those who are the wrongdoers.” (Qur’an 9:23)
“Islamic State member executes his mother in Syria: monitor,” Reuters, January 8, 2016:
An Islamic State militant executed his mother in public in the Syrian city of Raqqa because she had encouraged him to leave the group, the Syrian Observatory for Human Rights reported on Friday.
The woman in her 40s had warned her son that a U.S.-backed alliance would wipe out Islamic State and had encouraged him to leave the city with her.
She was detained after he informed the group of her comments, according to the British-based Observatory, which monitors the war through a network of sources on the ground.
Citing local sources, the Observatory said the 20-year-old man executed his mother on Wednesday near the post office building where she worked in front of hundreds of people in Raqqa, a main base of operations for the group in Syria.
The Islamic State group, which controls wide areas of Syria and Iraq, has executed hundreds of people it has accused of working with its enemies or breaching of its ultra-conservative interpretation of Islam.
For Reuters, you’re ultraconservative if you implement Sharia in the Islamic State and ultra-conservative if you oppose Sharia in the U.S.