The thing is that she suspected from the beginning while Batman et all have to chase down clues. It's Batman, not her, who tip off the cops. It's Batman who helps compile the evidence, not her.
And really, Batman isn't reponsible for the deaths Joker et all cause. It's the fault of the State who simply cannot be bothered to invest the time and effort to keep Joker locked up. Batman isn't legitmate law enforcement by any means.
Realy, Frank Castle's methods are shallow and weak, all about self gratification. I sure as hell wouldn't want him as a model for other heroes.