I've thumbed through the God Delusion book and have it on my list......my rather long list of books to get.....sounds like I'll add another to the list and........when I have time........sigh.
However, the reason (some) religious people behave in such appalling ways is because they believe their god requires them to do so. They're not extremists, they're people who actually believe what they say they believe. If we are willing to validate anybody's belief in any deities without any evidence (and unwilling to question or disparage such belief), then it should not be surprising that some of those people will actually obey their gods. Fortunately, in the civilised world, most believers ignore their god or have deliberately sought out (or invented) more benign deities but in much of the world, religious people follow their gods' commandments - well, religiously.
Eh......isn't God simply within and if we all wanted peace our God would be one of peace and the world would be a peaceful place. It's people doing bad in the name of their God. IMHO.
Peace and out.