These three are all pretty much hardened serial killers, but they each have different rhymes to their reason.
Kisame'll go pretty far to complete a mission, killing or mutilating you, and generally has a blood thirsty nature. But the dude also has a human side to him, he's not all about death.
Kakuzu's just uber efficient with a strong attachment to wealth, and a wicked temper. I don't think he'd outright leave and decide to kil the next person he saw for the sake of it.
Hidan's a religious zealot who literally worships a deity that demands self mutilation and sacrifice whenever possible, and not destroying your opponents absolutely is a sin. This dude is insane, has nothing to lose, and would likely go on a murder spree for the sake of it. He's by far the most ruthless and cold blooded.