They are Islamic, and such would be this way when using the term Heretics. Even others overseas think and say so.
In a telephone interview with AFP, one of the authors, Syrian-born journalist Hassan Hassan, said it was vital to understand that some of the group's core religious beliefs were widely shared. "It presents itself as an apocalyptic movement, talking about the end of days, the return of the caliphate and its eventual domination of the world," said Hassan, who lives in Abu Dhabi where he works as a researcher for a think tank.
"These beliefs are not on the margins -- they are absolutely mainstream. They are preached by mosques across the world, particularly in the Middle East. "ISIS takes these existing beliefs and makes them more appealing by offering a project that is happening right now," he said, using an alternative name for IS. IS uses certain texts and in-house clerics to provide religious justification for their violence, particularly a book called "The Management of Savagery", which argues that brutality is a useful tool for goading the West into an over-reaction.....snip~