Chuck, no I have not served in the Armed Forces. Does your question mean that for every other mass murderer, we can only understand if we have been:

orphaned
sexually abused
had drug addicted parents
lost family members due to murder
locked up in the home

I am very appreciative of this man's service and his mental state should absolutely be used as a mitigating factor in sentencing (if found guilty). But, it can never be used as an excuse for the act itself unless you are willing to let everyone else use any excuse that they have.