Radical ideas. They can overtake a human being. Sort of like a virus. Richard Dawkins invented the term "meme," to explain the phenomena. Some of the memes can be very toxic. Brains can go really, really bad. Especially in the realms of religion and politics. That's probably why in some homes those topics are forbidden at the dinner table.
These brains ravaged by memes can lead people to do some really heinous acts. We see it all the time. We spend lots of time trying to understand these people. Understand why they do what they do. But it might be as simple as some kind of zombie brain virus gone bad. The meme takes over. And the brain sort of hollows out, and the human being starts acting in weird ways.
Think of radical Islam like a toxic brain virus. Or Christian fundamentalism. Or Nazism. Or White Supremacy. Or Maoism. Or Capitalism. Zombie Brain infested people like to wield "isms" like machetes!
Toxic brain shit. Once a meme like this takes hold no telling what a human is capable of doing.