How so? Batman already suspected Branneck. He'd already clued in the cops, and they were hamstrung by their inability to prove it. In addition, they'd already caught the guy and let him go, on a technicality. Her "failing to inform" the cops of what they already knew and were unable to act on did nothing to make the situation any worse. And her action was the only thing that stopped the guy. The cops had their chance. Batman had his chance. They failed, because they followed the rules instead of doing what it took to keep more women from ending up dead.
That's why the whole "never kill" rule is a load of shite. Every time the Joker goes out and kills more, it's Batman's failure to end him that is responsible. Every woman this guy killed after Batman knew it was him, that blood is also on his hands.