Who was the man who shot Bill 'The Butcher' in the theater? I suppose he was priest Vallon's friend, but he wasn't shown before. And what was he saying while he was dying? Was he actually praying or saying something to Amsterdam in some other language?
An unnamed assassin
Who was he? Based on his attempt on Bill's life, and the cross he carries, it seems likely that he was linked with the Dead Rabbits, and was waiting for an opportune moment to kill Bill, possibly in revenge for the murder of Priest Vallon.
Was he praying? Probably. As a religious man he most likely was making peace with God. What prayer it was exactly, we'll likely never know. However, the Catholic News Agency has a list of Prayers at the Time of Death, so potentially the answer could be there.