But it wasn't just a 'simple' arrest, was it? Rightly or wrongly, force was dispensed during the arrest.
That part is what drew the brother in black. If they have police in their family, as I'm led to believe, do you not think the initial brother might have thought the action was heavy handed? Remember he did not, initially, strike the officer, but put his hand on him to pacify the situation of 'aggression'. The brother in black reacted after getting struck himself.
Look at it again, don't look at it. I'm not condoning how any of it played out.