Perhaps the radicalism of “mainstream” Islam is just inherent in its nature. Mohammed wanted a monotheistic religion to replace the old pagan one for multiple reasons. First, to compete with the others, making it a particularly antagonistic one. Another, to recruit fanatical troops for his Jihads of conquest who weren’t afraid to die, being promised not only sex slaves in the afterlife but mansions with servants, possessions, etc.
And the moderates, saying the God of the Bible is the same god as the Muslim one, aren’t really being true to their own belief, since they claim the Bible was corrupted and is basically a lie. Convenient, since there are so many contradictions with the Koran. But what do we know of the author? His visions came from epileptic fits. He wrote a set of books that later had to be edited from the Koran because they commanded his followers to worship the daughters of the pagan god Hubal. He was a warlord who purchased a 6-year-old wife. He had a penchant for murder. Yet he is compared very uncharacteristically to one of the Biblical prophets.
The Koran even contradicts itself in places, and is arranged haphazardly. And unlike the other major religions, Muslims are not allowed to research their religion too closely. There are religious police in Muslim countries that punish any open critique. Does something that is true fear the truth this much? Why is a former pagan shrine the most holy site in Islam? If Mohammed deserves one compliment, it is probably that he is the most successful false prophet in history.