Good analysis.No Hidan kills for his beliefs just like everyone else in the manga. That aint cold blooded, it's his faith/religion. It's like the christians of old going on a crusade.
Kakuzu is the worst, he does it just for the doe or kills his partners jus coz they irritated him one time.
Hidan is second (despite his faith, it is a twisted religion)
Kisame is the least cold blooded. He showed remorse for killing his comrades but did it to protect the secrets of the mist.
Hidan ironically is not cold blooded. He loves Jashin (even though Jashin doesn't exist). He kills for somebody he LOVES. Jashin commands him to kill so he does similar to Kimimaro. Kakuzu on the other hand kills for money. He kills his comrades cuz he got short temper. He is a survivor imo. He's mean and cold blooded.
I'll rank him as the most ruthless followed by Kisame.