Honestly, after reading Batman #422, I'm shocked that this comic isn't brought up beyond the panels of Jason beating up an abusive pimp. Like, only two issues before Garzonas dies (or is killed), Jason is part of a case involving sexual violence, the perp walks free and is going to leave Gotham to hurt other people, and the rapist and murderer is only stopped because someone kills him.
Jason isn't the one who kills Branneck, but he doesn't disagree with Judy's actions either. He sees firsthand that if Judy hadn't killed Branneck, another woman would have died. There was nothing Batman, Robin, the police, or the law could do.
I've got a longer post in the works (it still needs image descriptions), but this is, like, the context for Garzonas. We see Jason acting uncharacteristically violent in response to a rapist and murderer evading justice, we see Bruce and Jason discuss the righteousness of killing such a man, and we see directly saving lives that Batman and Robin couldn't save by killing Branneck. Whether you're wondering why Jason might have killed Garzonas or why Bruce might haver believed Jason did it, this comic has answers.