Hi Lonelyone: One situation I can think of that may cause a person to overreact: if he thought you were downloading illegal porn onto his computer, and he feared he could get in trouble with the feds. yes, that could cause someone to go ballistic. if he had other things that he was afraid of compromising, and this scared him that much, he could lose it.
I would check into his family past.
One of my friends is not violent or alcoholic, but because he had problems like that on his father's side, it shows when he gets impatient would beat up on objects, like hammer the heck out of a lock for getting stuck. He wouldn't hit on people but he would slam objects when we know logically that doesn't help. So his tendency would show but he kept it contained. Maybe your BIL has some issues in the family past that come out during stressful triggers.