But does River kill the killer though? I don't think he does, even if you decide you want to to help him (he declines your help and he didn't end up killing Anthony from what I can remember from my playthrough).
He does not kill him. The point about is that in normal game or any other RPG declining quest leaves characters and events waiting for you. If you decline to find golden claw in skyrim quest will be always waiting for you there and no one will find it. In C77 there is a lot of "nope" where you can decline and world moves on.
The further point about it is that game fully acknowledges that by having that second River conversation at family dinner. Normally in such games at best you would get some quest failed detail or some offscreen info.
River quest line also shows a lot more gameplay oriented choice rather than pure C&C like TW3. For example minefield could be switched off via hacking, or via computer in attic, you can go through it or you can pick up mines with gun. There are also high checks for hacking in there.
Gameplay aspect is major upgrade over TW3. But this was at the cost of more railroady quests. Overall it is obvious that they had to put a lot more work into C77 quests than in TW3 ones. Cut corners is obvious outcome when you had limited amount of people and you have to take into account various ways in which people can approach quest and not see ever FAILED BECAUSE YOU DID IT WRONG.