He can still take the stand and admit to what he did and why he did it if he wants.
Bottom line is it doesn't matter because the Army's case is overwhelming, there were so many witnesses.
In a narrow sense, perhaps, but in a broader sense there is more than Hasan on trial. An unsuccessful defense based on the murderous tenets of islam would have a chilling effect on some future jihadists (spell check wanted to correct that to "sadists"!) and establish a cultural limit on that 'up with which we will not put'.
Repudiating islam as an excuse for mass murder has its own value, wouldn't you agree?
Or are you really afraid the court could find that it actually is?