Of course everyone deserves a fair trial and that one gets the punishment that suits the crime or no punishment if no crime has been committed. If not, then it would be left to mob rule and that fella would be lynched on the spot.
The issue, vis-a-vis Harley, is not that he would represent this person, or any person, that the public has deemed a scumbag. A more interesting question for Harley would be that when he represents a client what takes priority? That his client gets a fair trial or that he wins his case?
Let's say Harley did get this person off on a technicality. How would he view that? Would he feel justice has been done? Would he feel that he has made the world a better place? When he is on his death bed and so many of the things he thought were so important in life start to fade into the background -- will he look back on this victory with a sense of pride? That he did the right thing?