It seems rather simple to me.
The killers just wanted to kill and the film was a convenient excuse to do so. They bombed a German embassy because of an American film! This just shows that they wanted to attack non-muslims anyway regardless of the film.
Moreover, other cultures get insulted in films all the time! Why should they get a license to kill when their culture is insulted?
Bottom line is this: If thier god was real then they wouldn't have to defend him, he'd be able to defend himself. As Christians we serve the real God who doesn't need us to violently defend him because he said "Be of Good cheer I have overcome the World" (John 16:33)