If he survives, the Ft. Hood shooter will of course be charged with murder, but it's reasonable to inquire whether treason should also be charged. After all, for a major in the U.S. Army, trained at taxpayer expense in the use of weapons, to shoot 40 unarmed comrades-in-arms would seem like a reasonable example of waging war on the United States.
However, the Constitution's delineation of treason might not cover this:
Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort. No Person shall be convicted of Treason unless on the Testimony of two Witnesses to the same overt Act, or on Confession in open Court. The Congress shall have power to declare the Punishment of Treason, but no Attainder of Treason shall work Corruption of Blood, or Forfeiture except during the Life of the Person attainted.
What does "levying war" mean? Although "levying" is sometimes today said to be the same as "waging," that doesn't appear to be the legal definition. In one of the the treason cases (Bollman) growing out of the still mysterious Aaron Burr conspiracy, the Chief Justice John Marshall of the Supreme Court ruled in 1807, "But there must be an actual assembling of men for the treasonable purpose, to constitute a levying of war." In other words, "levying" means raising a body of warriors. Therefore, whether Major Hasan plotted solely alone or was conspiring with others, and if so, did they in some fashion "assemble," would appear to be relevant.
On the other hand, the second type of treason, "or in adhering to their Enemies, giving them Aid and Comfort" would appear to be an easier hurdle to leap. The first time the Supreme Court upheld a treason conviction was in the 1947 Haupt case in which naturalized citizen Hans Mark Haupt was sentenced to life in prison for sheltering in his Chicago home his son, a German spy (one of the eight saboteurs landed by a German sub in a semi-farcical failed infiltration). The son was convicted by military tribunal and executed. In the father's case, noted civil libertarian Justice William O. Douglas wrote the majority opinion upholding the father's conviction, while Justice Jackson wrote a lonely dissent arguing that the father's intentions were filial rather than treasonous.
Since the elder Haupt was legally guilty of treason for merely helping his son, then Hasan's shooting two score American soldiers in cold blood would appear to be an even better example of "adhering to their Enemies, giving them Aid and Comfort." However, that does raise the issue of who exactly our Enemies are, a question that has been left rather ambiguous by Congress' refusal to issue a Declaration of War since 1942.