As far as I know, there is no information publicly available about either the nature of Santiago's war experience or the nature of the mental health care offered to him in Anchorage during 2016.  Relatively few soldiers even in a war zone actually experience combat.  And it seems, at least at this point, quite possible that Santiago refused any treatment that may have been offered.  

More fundamentally, there is an obvious fallacy involved in listing, as Jim Gottstein does, the medications allegedly taken by notorious mass killers as if that proves the meds caused the killings.  Many millions of these prescriptions are written each year without, apparently, resulting in criminal acts.  And they are especially likely to be prescribed for people already suffering from some type of mental illness.  So it's not very surprising that some cases of violence are going to correlate with them, the way some deaths from cancer are going to correlate with chemotherapy.