Not being a mass murderer myself, and not knowing any to ask personally, I'll repeat what I have read (probably on the internet, so it must be true!). Those who are killing in the name of Allah think they have a religious obligation to rid the world of infidels (non Muslims), either through forcing conversion or killing them. Children, are better off dead than growing up to be infidels by choice. They believe the children are innocent and therefore, killing them sends them straight to heaven. I actually think I read that in connection with one of the Israeli/Palestinian suicide/murders involving children. Where do they come up with beliefs like that while other more moderate Muslims deny those obligations exist?
Well, read through the book of Judges sometime. Jehovah ordered the Hebrews to slay entire cities of pagans: men, women, children, born and unborn, and even their animals. Does that mean we should be killing unbelievers, too? HIS ways are incomprehensible to us.