I'm not sure I'd classify anyone here as trying to shield him behind anything. Until all the facts are in, then by definition, rationalization cannot even occur. The correct term, rather, is speculation. To rationalize is to ascribe one's actions, opinions or motives that, at least on the surface, seem plausible and credible, but in reality are completely unrelated to the actual deal, with the actual deal often being less palatable, less agreeable, less creditable. So, until we actually know the actual deal, the real and genuine bona fine root cause of what he did, everything is pure speculation.
Trying to come up with (speculation) the reasons (A.K.A. the required-for-conviction motive) for what he did is not even close to shielding him behind those reasons. Motives and reasons are not the same as excuses, and even if they were, no excuse can excuse what he did. There is no cut-off point for responsibility, since responsibility is instantaneous.
By the way, for those who keep wanting to separate out "snapped" and "planning" as somehow being mutually exclusive, they're not. Snapped doesn't necessarily mean spontaneous. Snapped is a euphemism for "crazy". People can, do, and have "just snapped" and stayed "just snapped" for days, weeks, months, years. And all the while they are "just snapped" they are sitting there planning something that has "crazy" written all over it.
Most people would agree that there is a difference between a terrorist committing a terrorist act against an innocent public for political, philosophical or religious purposes, and in a disgruntled worker Going Postal on his co-workers. If you look at it even semi-objectively, what Nadal did at Ft Hood is much more akin to Going Postal than a terrorist act. Shooting your co-workers in an enclosed space at Ft Hood is very different from going into downtown Killeen and shooting up the Court Square. The latter would provide much more terror among the general population, and would almost certainly force the government to alter the way that military base is operated (both of which is what modern-day terrorism is designed to do). The former doesn't do any of that.
Regardless of the motivations behind what they do, the one common underlying factor among all shooting rampages is, the shooters are all crazy. Every one of them, without exception.
"Just one man killing indiscriminately."
There ya go. Exactly.
) ..... but the apparent premise that committing evil towards one fellows is somehow not insane is false. Completely and utterly.