The extremist Islamist group currently wreaking havoc in Iraq and Syria, which has prompted US airstrikes to prevent a potential genocide, was even too brutal for the mastermind of the September 11 attacks, Osama bin Laden.
According to a report in the Daily Mail, among the items found in the Pakistan compound where the former al-Qaida leader was killed by US special forces in 2011, was a 21-page letter warning of the dangers of the Islamic State, formerly known as ISIS.
In the letter, Bin Laden reportedly called for al-Qaida to sever all ties with the group due to its extreme brutality.
He added that ISIS's blatant disregard for civilian life was likely to damage the reputation of al-Qaida should the groups remain associated.
The letter was not written by bin Laden himself, but by a senior al-Qaida official who worked in his service, according to the report.
Making the case for why al-Qaida should distance itself from ISIS, the Daily Mail reported that the letter listed a number of atrocities committed by the group, including the use of chemical weapons, bombing mosques and massacring Catholics in a church in Baghdad.