As points out, Batman only targets Branneck by chance -- if he hadn't spotted Cutter at the fire, it's unlikely that he would have followed the trail that eventually led to Branneck. Plus he only really started on the case in ernest with victim 5, and didn't cotton on to it being Branneck until it was I think victim 7.
The sister had decided it was Branneck in the wake of only the 2nd murder. That's at least 5 deaths she's morally culpable for by not coming forward with Branneck's name to the police but instead choosing to keep it secret so she can be the one to kill him in revenge for her own sister's death.
As an aside, if they let the sister walk on this, I don't see why Batman was such a hardarse over in The Hiketia.