can anyone explain this to me?

How does anyone consider themselves to be religious, and acting in concert with their beliefs, and condone the mass murder of people on an airplane....or running airplanes into skyscrapers....or murder in general, in the name of their beliefs? There are many reasons people kill people, and not many, if any, are very good. To do it because of your religious beliefs though, and to make a point of killing as many as possible...how does anyone justify that?