I loved the movie, now for the ramble and the critic....why on earth did the protagonist keep releasing the bad guy and giving him more chances to r*** and kill more and more people? Another thing that was strange to me was how robotic the victims of the antagonist were... For example the second girl who jumped in the car with him for a ride and just died? She didn't even raise her arms for protection which was odd to say the least, the old doctor taking the humiliation like it's nothing, the girl at the p******* seeing this weird customer in the employee area and not even screaming, the pharmacist putting up with the disrespect before getting holed in the neck, everyone is just *there*
Been a while, since I watched this, so I can't go into the scenes you described, but it's one of my favorite revenge-movies, so I'll try with the character's incentive for the catch-and-release.
The whole point - for me - was to emphasize, how the protagonist is consumed by revenge. Ignoring the fact, that his revenge-fueled hunt not only gives the killer more chances, but probably causes more death and pain. As in, he saw the devil and it changed him so much, that he can't see, how he is partially to blame, for anything his freshly released killer does. Or he does see it, but does not care (prioritizes revenge; lost that part of his humanity already?).
From the protagonist's POV, simply killing him for revenge, would not suffice to still his hunger. It would be close to a mercy-killing in his mind, putting a bad dog to sleep. He needed the killer to feel like prey first. To make him vulnerable; run away scared; torture him with a hunt, instead of just ending his life, or inflicting pain.
We could guess, why the character fails to see his own fault. But I think, when it comes to revenge, you can't filter a character's way of thinking with rationality.
There are many good quotes that describe this character flaw. Like Nietzsche's "*He who fights monsters should see to it that he himself does not become a monster"*, which is exactly what happens here. Or to stay in movies maybe a better known one from 8MM (1999) *"If you dance with the devil, the devil don't change. The devil changes you."*
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