This is a very primitive and counterproductive action.
You defeat people most optimally and effectively with words (arguments) and if the opponent's claims are outrageous it is even better to ignore it completely as in this case.
Not only that, this 75 year old grandpa is no longer a Muslim, he does not live in a Muslim/Islamic state hence useless 30 + year old fatwas from Mullahs in Iran don't apply to him. Hence not surprised that the criminal who attacked him was a Shia as many of them tend to worship whatever the Mullah's in Iran tell them to do.
Even in a perfect Islamic state that does not commit any wrongdoing and where solely justice reigns supreme you cannot make such kind of fatwa or condemn a former Muslim unless he has renounced his faith in front of a Islamic court 3 times in a row (if I recall).
This will just lead to greater Islamophobia, more potential attacks against innocent Muslims in the West, more bad publicity etc. in short nothing good will come out of this at all.
Not only that, the guy is still alive so the attacker could not even finish his job as he wanted to. Pathetic.
Those people who support this primitive action are the actual anti-islamic people.
Let Allah (swt) deal with him, he is the best judge after all and his eventual punishment will be far greater.