I'm sure from a legal stand-point, she'd be able to argue a valid case for shooting Batman in self-defense; he's an armed vigilante, he's just been beating up a man innocent of this particular crime (let's face it; describing PENGUIN of all people as an 'innocent man' is slightly questionable) and he's probably armed up the wazoo with all sorts of stuff that could theoretically dismember her. If it came to court, I expect that Romy would be able to argue her way out of the charge with some reasonable arguments.
But that doesn't change the fact that she allowed her emotions to get away with her, and fired a kill-shot at a man based largely on the fact that she extremely disliked him, without really giving him adequate opportunity to comply with her requests. A man who, let's remember, even with his then-current difficult relationship with the Gotham PD had never picked up a rep for violence against police officers. I mean hell, she even pretty much ADMITS it in later panels that she fired on him because she hated him. It's frankly a bit disingenuous to talk solely about her legal justifications for doing so without acknowledging this point.
Batman may be a bit of a dick here, but let's face it; in this situation Romy's hardly the pure-as-driven-snow angel making a righteous shot that some seem to be making her out to be either. Which, incidentally, is one of the reasons I loved loved LOVED this series.