Muslims: how do you view extreme islam?

Being a Christian myself and knowing that Islam and Christianity share many similar characteristics (Islam recognizes many of the Christian prophets, for example), how are extreme islamists (who most Americans know simply as "turrorists") viewed by both sunni and shia muslims? Extreme evangelicalism is about the most extreme form of Christianity I've seen, but are the so called terrorists really religiously righteous? Is jihad technically justified? Or are they simply crazy/motivated by something else?