Those four S-rankers were more intelligent and likeable than other criminals that Rio hunted down, but they were ultimately still horrible criminals in the end.
Each one of them had committed an uncountable number of crimes and bad things.
He already knew most of their information, as Irvin had already provided him with enough proof to consider them guilty.
Rio is not cruel enough to torture innocent people like this; moreover, he does not even consider his way of carrying out his missions a torture method.
The main reason why he's even doing this is because he wants to test out his skills and improve them.
Confronting many S-rankers brings him new inspirations, enriches his combat experience, and provides him the opportunity to learn new skills and techniques.
This is basically him treating his opponents as test subjects; there's nothing else to it.
'I swear I am not sadist at all… Yeah, it's all just for the sake of testing my new skills, yeah…'
