Why do extremists yell out 'Allah Akbar' (God is Great) before they commit an atrocity?

Allah-u-Akbar means "God is great." Muslims say that when starting a prayer, when they are in danger, making a huge effort, or fighting. The idea is that no matter what a person does, whether there is success or failure, victory or defeat, God is ABOVE all that and His Glory is not tarnished.

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