In all likelihood, the SSGT in question had severe PTSD, and though he had repeatedly complained of it to his chain of command, it was either ignored, or he was accused of malingering.
I suspect that he had either an Article 15 or even a court martial pending. But this will only be known if his command doesn’t immediately destroy this evidence.
However, it he also complained to an honest chaplain or medical officer, it is a wholly different situation. There will be a huge investigation and a lot of people will face punishment.
As far as the SSGT goes, once properly evaluated by a psychiatrist, he will be shipped home and have to undergo intensive therapy and rehabilitation. Any military criminal punishment, oddly enough, will be mild, as what he did does not seriously interfere with morale and discipline.
There is an additional provision however. Not too long ago, congress passed a horrible law that made military personnel subject to civilian prosecution for acts they may have carried out overseas, after they have left the military. So if he is not prosecuted by the military, he may face an ambitious, anti-military prosecutor.
Ironically, I had come across a link a few days ago to a story about Sam Ybarra and the disturbing actions of Task Force Oregon (a.k.a. "Tiger Force") in the Central Highlands of Vietnam in 1967. Ybarra, who by all accounts had pretty much gone off the deep end, was never prosecuted by the U.S. military ... and most of the details of the story came out years later as a result of some research and investigation by a newspaper reporter.
And certainly there's no shortage of them around.