If Billy gets beaten by his dad in all probability Mark did too. Perhaps that is why he went away to uni to escape the violence. He says he is a recluse which could be an effect of the beating but most likely he will feel guilty to his brother for leaving him at home on his own with the man who beat him.
He will no doubt feel responsible for his brother commiting suicide and as a result takes his anger out on the security guard who actually had very little to do with the suicide but was just the straw that broke the camels back.
If Billy gets beaten by his dad in all probability Mark did too. Perhaps that is why he went away to uni to escape the violence. He says he is a recluse which could be an effect of the beating but most likely he will feel guilty to his brother for leaving him at home on his own with the man who beat him. He will no doubt feel responsible for his brother commiting suicide and as a result takes his anger out on the security guard who actually had very little to do with the suicide but was just the straw that broke the camels back.