I'm guessing they wouldn't be outright sadistic, but would still have a "life is pain, deal with it" attitude.
Yeah, but they could focus on darkness/shadows, kind of forget about the sadism. Could be a surgeon, that enjoys process a little too much. Could focus on Intimidate, just as easily, too.
If you are confused on how to play an alignment, I enjoyed watching this video and this video. Both kind of talk about how to weigh out each viewpoint of alignment. For me though, unless I'm playing a Paladin or a Cleric with a truly strict god/goddess, I don't stress too much on alignment.
I would play the Kuthonite Cleric like this:
They respect law, or their own personal code, or people in authority.
They are dispassionate when it comes to either supporting the personal opinions of those who care for others, and those who care for themselves. As long as the law is upheld, they wouldn't care who wins out. When the subject is up in the air and there is no clear definition, they can make up their own mind (usually on the side of evil regarding matters of pain or aspects of Zon-Kuthon, and perhaps good on other matters).
One thing I like to keep in mind is that even evil characters can donate to charities, have people they love, and can be family to people they care about. They just do it for selfish reasons usually. Just because you follow an evil god of pain, doesn't mean that you are bound to be a mass murderer hobo. You are a functioning member of society and not necessarily a villain (unless you want to be). In fact, in your sight, you may actually be good! Keep in mind that pain and murder might be justified in your moral code and may even be seen as good to you. Loss and death are just stepping stones. Perhaps you kill those who are weak to help the strong survive. This is a proven fact in nature that helps the world survive, you can just apply this to people who get in your way too.
I would play it with a more monk-like mentality, like the Cenobites; seeing pain as a gateway to a transcendental state, subjugating the body to enrich the spirit.
Personally, Neutral clerics of evil gods or vice versa for good gods don't make any amount of sense to me. It's like donating to a mass murderer because you think they're a swell guy when they aren't murdering people.