Speculation is a disease in journalism - the Daily Mail did not quote any reliable sources and only relied upon conjecture. The same source of the story is a person who is actively fighting against the group he is defaming.
There's bias from the source, and there is bias from the tabloid - as much as I may not agree with all the acts that are attributed to ISIL, I cannot believe that they committed Z because they have committed X and Y, unless there is substantial evidence for Z.