It's an assymetrical war. We did the same thing gaining our independence (breaking the rules at the time).
The fatwa by Osama Bin Laden says he's killing our civilians because they pay tax money to an industrial-military, crusader-zionist alliance that imperializes the middle east.
They're not irrational people. They're angry and they know they're weak, so they spread the force they know and hate: fear.
At the end of the day, terrorists are utilitarians, believing the ends justify the means.