It's not so much "religious" fanatics as just plain ole fanatics. Some people are just cold in thier spirits and become calloused to caring about other people in general making it easier for them to commit horrible acts against others. Some afiliation with an organized or named religion may or may not have influenced them to act in such a heartless manner so we shouldn't focus on the religion but on the fanatic.
Just a guns do not kill people... killers kill people. Whether by gun, knife, rock, bare hands, rope, stick, bedsheet, scissors, box cutters, ball bat...well you should know by now the list could go on forever. Religion doesn't kill...killers do!