:bism:
This is a very simplistic worldview. Prophet Musa/Moses alayhis salaam (peace and blessings be upon him) still had to do dawaa (invitation to submission to God) even though Pharaoh was one of the worst examples of humanity in history because God still held Prophet Musa/Moses alayhis salaam (peace and blessings be upon him) responsible for conveying the message. In that same way, the terrorist organizations and extremists are evil, misguided, and deluded. However, as laypersons, we're not allowed to say that they have become disbelievers simply because the only one allowed to do that are Islamic scholars.
That said, Islamic scholars have taken the position that the extremists' and terrorists' actions are un-Islamic and haraam. Also, Islamic scholars have deemed them as possessing the characteristics of Khwarij as they often perform takfir (declaring Muslims "non-Muslims") which of course means that takfir reverts back to them, and they are the ones who instead become disbelievers in the eyes of Allah (God). So, all in all, as laypersons, all we know for sure is Allah (God) will judge them for their evil and they will be paid in full for their evil actions on Judgment Day.
Muslims worldwide are against extremists and terrorists. Full Stop. Period. Dot. However, whether the extremists and terrorists are out of the folds of Islam, is not the judgment domain of the Muslim layperson but Islamic scholars. So, calling an extremist or terrorist a "brother" is not actually endorsing the extremists' position.