Batman IS legally bound by the law. He just chooses to ignore it. There is a difference, and that difference means it is occasionally perfectly justifiable for someone to shoot you in order to stop your assault on another person.
Let's put it another way. If Batman was crouching over a sobbing, terrified woman who he had clearly beaten, saying he wasn't going to stop beating her, would ROmy have been in the right to shoot him? Hell yes. That doesn't change. He was assaulting someone in an illegal manner. He refused to stop. He gets shot.