I agree that Photo Finish feels like a more clear cut case, but that reflects good decision-making on the part of the author more than any definitive ethical principal. If the murderer had appeared grief-stricken and confused, if the woman had been a yuppie in a business suit, if the photographer had had an amputated leg, if any number of details had been different, we might be siding with the photographer. If, on the other hand, the woman had been an eight-year-old, if the murderer had curled his lips into a lascivious smile before driving off, etc., the opposite would probably be true. Yet in all those cases, the ethical nature of the photographer's decision remains fundamentally the same.
As it stands, we really can take both sides and I think that was a very deliberate decision. Not because the story is meant to be ambiguous, but because the justice and injustice play well off one another. The fact that he deserves his fate makes it gratifying while the fact that we identify with him, even like him a little, makes it horrifying.