It is called Islamic extremism because that is what its perpetrators call themselves.

They are no doubt social misfits with serious mental problems, but when such social misfits act in the name of a particularly extremist version of a religion, the ideology is involved, too.

Just as those misfits acting in the name of Marxism were called Marxist terrorists, and those terrorists acting in the name of the F.L.Q. were called F.L.Q. terrorists.