From Cerberus files, Ch 1 author’s note:
The Sole Survivor is a broken figure, losing his family, his unit, his fiance and finally his sanity and becoming a terrorist threat to rival Cerberus, convinced only he understands the sacrifices needed to bring humanity to a shining future – of human dominance.
He’s been horribly injured in the aftermath of Horizon, so is this where the sanity loss comes in?