i think he kept killing the quintuplets because they all look the same, constantly reminding him of his first murder, hence the END message, partly because he also wanted to stop killing. but he kept getting killboners in between all these, and the other victims were just unlucky he noticed them when his killer instinct manifested. someone said that the shrink dude could also be one of the quints and may have had a hand in brainwashing people into being murderers including Masato, but honestly I'm starting to think this manga's writing is a bit too contrived if that conjecture turned out to be true. Also, still zero leads on the policewoman's dead brother and his lookalike (who also happened to be acquainted to the shrink/shrine-priest), too much emphasis has been placed on it for it not to have any link to these murders.