I'm afraid there is a fair precendent of people going on a bloody rampage in the US (and elsewhere) without a connection to Islam. Islamic extremists don't have a monopoly of violence in any context.
To my mind, he may have come to the opinion that the wars in Iraq and Afghanistan -or their implementation were unjust, or something he wanted no part in. As a civilian, he might have been a conscientious objector perhaps. It seems he repeatedly resisted posting to either war and in failing to avoid going, presumably he simply couldn't take it and snapped.
As a soldier, he had free access to the fort, training and presumably easy access to weapons.
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