This sounds like a classic example of  one result of a brain /head injury.  Absolutely of course not all head injuries result in sociopathic behavior. 

I read of an extensive  study of serial killers and  a sizable percentage of them- 2/3rds?-had suffered a serious head injury that altered their personality and behavior from that point onward.  That is the key-altered the core personality profoundly. I imagine most head injuries heal with little to no lingering after effects. If he suffered serious abuse there’s a chance he took some hard blows to the head.

Perhaps you could find out if this occurred with you SO.