He snapped after witnessing too much, and having a friend's leg blown off by an IED. How about those who place the IEDs that also kill civilians and children, or the suicide bombers who do the same, but they can't be revenged upon too easily. Serial killers? Afghanistan is full of them.
Personally, I don't think we should have ever sent troops there, making the same mistake the Soviets made. The known terrorist sites targeted with cruise missiles of little effect by Clinton in '93, and any other sites known to train and harbor terrorists, should have been nuked on 9-11 and 12 , 2001. Places that had cheering crowds of terrorist supporters should also have been nuked.
The majority of the world would have been shocked , but done nothing, because it was retaliation for an attack, war not started by the US. Ground troops should have been put on the Mexican border instead, to fight the de facto long time invasion of millions from Mexico, that has now killed around 100K Americans with little fanfare or publicizing, because it was slower, and by several means (murders, ODs, various diseases brought in, drunken driving, negligent homicides, etc).
Borders closed, and Americans allowed to tear down mosques, moslems deported. None of their kill cult ilk should ever have been allowed in the USA. Madison said, in his papers on the Constitution that freedom of religion was not meant to include mohammedanism or other Eastern religions. At that time, the moslem religion had declared holy war against the US and never rescinded anything said in their book about killing all infidels or enslaving them and making the whole world an islamic caliphate under Sharia law.
See the White Roses links to educate yourself on just what a horrible thing islam truly is. Ignorance has prevailed since the days of Madison and Jefferson. I remember the chicken Cassius Clay turning into Mohammed Ali and refusing the draft saying the BS that islam was a religion of peace. Taqiyya makes it so liberal ignoramuses fall for it, much to the moslem glee.
I am surprised there aren't more who have opened up on them after experiencing their atrocities. Islam is like a disease and should have been contained to the Middle East. Their main method of conquest now is through taqiyya with immigration and force breeding their females to have 8 kids to eventually outnumber their hosts.

Before every action and decision think of the consequences 7 generations into the future....Ute Rule of Life