This is fringed thinking, but, is there a remote chance the younger brother was roped into this unwittingly. Was he asked to help his older brother with some kind of delivery. The bags were left outside an establishment. He reacted to the explosion. Could it be he didn't know until that moment what occurred with the bag he was asked to leave on the street? If he ran over his brother, it makes somewhat sense he was not invested in the understanding of his mission, almost as though he was trying to get away from his brother himself. 

That is really fringed thinking, but, it is remotely possible.

Yep, a You Tube posting. The older brother was demented into believing a holy war was his destiny. Unfortunately he is dead now.