According to this news story, the U.S. soldier in Afghanistan accused of a brutal, murderous rampage has suffered a mild traumatic brain injury in the past. It's not clear from the piece, however, that it played any role in his conduct:
An official told ABC News that the soldier has suffered a mild traumatic brain injury (TBI) in the past, either from hitting his head on the hatch of a vehicle or in a car accident. He went through the advanced TBI treatment at Fort Lewis and was deemed to be fine.
He also underwent mental health screening necessary to become a sniper and passed in 2008. He had routine behavioral health screening after that and was cleared, the official said.
When the soldier returned from his last deployment in Iraq he had difficulty reintegrating, including marital problems, the source told ABC News, .[sic] But officials concluded that he had worked through those issues before deploying to Afghanistan.