I'm totally for killing the worst of the worst, but I think that this would be disasterous for Cass for 3 reasons:
1. Like jlbarnett said, killing is repulsive to her (even though some writers conveniently forget that from time to time)
2. Even though killing horrible monsters like the Joker is a sensible thing to do, pretty much everyone in the DC universe would instantly condemn Cass. In the DC universe, killing usually = bad, so writers would probably make Cass out to be misguided and morally wrong. I'm next to positive that if Cass took this route, DC would completely trash her as a character.
3. I really, really, really want Cass to keep being Batgirl :)