Nidal joins the military. When? 1988 according to Wikipedia.
Nidal Malik Hasan - Wikipedia, the free encyclopedia
1988 was a while ago. From memory there were no hostilities or engagements between the US and any Muslim nation or terrorist organisation. As always, I'll ask for correction. Thus it would seem at that time that there would be nothing being done by the US Army that would anger a Muslim soldier.
Again according to Wikipedia, Nidal became more devout after the deaths of his parents in 1998 and 2001.
I'm wondering if Nidal became more and more devout and treated his religion as a shelter from what was happening around him at the personal level. It appears that he wasn't married (which can contribute to personal stability) and didn't have children (which can contribute to a sense of purpose and responsibility in life). And if that is the case then it's probably not surprising that he sought out Islamists, not the other way around.
His behaviour, again described in Wikipedia -
Nidal Malik Hasan - Wikipedia, the free encyclopedia - sounds like that of a person who has decided to commit suicide. He gave away furniture. He also gave out copies of the Qu'ran, although it's not known to whom. My understanding of the Qu'ran is that it cannot be touched directly by non-Muslims so it might be that he was giving out paperback copies rather than the book itself. I'm no shrink but this behaviour taken as a whole might indicate that he had decided to kill and that he expected to be killed.
We'll know more later. Right now we're all speculating, but as always it's interesting to do so.