My friend, don't put any precondition to respond physically. It is neither legal nor spiritual to rspond physically unless it is for self-defence. Otherwise learn to forgive or use legal means in the worst case. You defend urself and let God defend ur faith/religion. if somebody says bad things about ur religion plz don't respond physically whatsoever. Just leave him and let God judge him. this is what I think the muslims lacks.
As a Muslim I completely condemn this act. As long as the guy is not attacking (verbally or psychologically) the Muslim community , no body has a right to lay a single finger on this "Mohamed" guy. There is no Sharia system in Ethiopia to judge some one for "Redd"