The 21-page letter, written by one of Bin Laden's senior officials in 2011, claimed the Islamic State (IS), formerly known as the Islamic State of Iraq and Greater Syria (ISIS), has complete disregard for civilian life, the Daily Mail reported.
According to the letter, the IS's barbaric acts include the use of chlorine gas as chemical weapon, bombing mosques, crucifixions and beheadings.
The letter said that the IS was simply too extreme even for the group that killed thousands in the 9/11 attacks.
Today, the IS has become a powerful force that has control of areas in Iraq and Syria larger than Great Britain.
Its trademark black jihadi flag has fluttered in the background of "promotional" videos of executions, including crucifixions and beheadings.
The 'caliphate' the Islamic state has claimed to have established, represents the biggest shift in the political geography of the Middle East since the borders of modern Iraq and Syria were drawn under the Sykes-Picot agreement between Britain and France in 1916.
The IS was founded by 43-year-old Abu Bakr al Baghdadi, a cleric held prisoner for four years by American troops in Iraq.
The leader, a Sunni Muslim who despises the Shia-run Iraqi government, now commands more than 10,000 fighters, many of whom are former Saddam Hussein-era soldiers or disenchanted Sunnis who lost power and influence after the fall of the dictator's regime.
The group is estimated to have amassed a staggering £1.2 billion. It has even sold 8,000-year-old antiquities it has seized.
The group uses social media platforms like Facebook and Twitter to promote its jihad by posting videos showing it killing soldiers and government officials.
Many places in Iraq, including Tikrit, Mosul and Erbil, are still in the possession of the IS.