Well that's a rather flimsy excuse for Cass stop being Batgirl... I was hoping she'd give it up because she'd feel wracked with guilt over the killing she'd done while wearing the symbol... as if she'd dishonored what it stood for.
She'd be wrong to think that way of course, but it'd be a perfectly reasonable human reaction to have. And Cass (pre Beechan) seemed to be all about taking responsibility for her actions, even things that were out of her control.