First, juries make mistakes all the time. Second, the jury did not find him innocent -- the jury found him not guilty. There's a big difference.
In MA (and in most states), you can only use deadly force if you, or another innocent, are in immediate danger of death or grave bodily injury. Was he in danger? Yes. Was he in immediate danger of death or grave bodily injury? Based on what little I've learned about the case, it does not appear to me that the situation reached that high bar. I think he is very, very lucky that he was acquited.
I strongly suspect that they suspended his license after the incident.
That's self defense law, not just in this state but in most states. I strongly suggest that you read Andrew Branca's book, the Law of Self Defense. Right or wrong, across our nation the law places very strict limits on when you can use deadly force. Note that in most states you can use deadly force to stop an arson of an occupied dwelling. I have not read anything that suggested the mob was considering arson.