The mob was rioting and damaging property before they were shot at. That shooting on top of the godhra train burning and massacre of Hindu women and children and pilgrims pushed the Mob into killing him.
Its common sense.
No one comes into your home at random and hits you on the head with a stick. Not unless they have good reason for it.
And those reason are usually money or property. Unless its about a women, but considering he was a muslim, it was definitely not about a women. Muslims do not go around killing each other in honor of their own women.