The Stand question

Remember, too, that when Nadine relates the story of having met "Joe" he is not exactly open and accepting of her either. Both Joe and Tom are meant to show the latent psychic abilities that most people have. Joe's abilities come about after the death of his family, when he regresses into a feral, animalistic state. He might have actually had some real hesitation about Nadine, but she was the only living person around and she was conditioned by her job as a teacher into sticking by and helping those children in trouble. He bonds to her and likely he isn't able to "see" the danger that she will represent. Whereas he meets Harold much later, after he has re-integrated some of his previous characteristics as "Leo". And Harold, unlike Nadine, is willing himself to hide his anger and resentment and rage. Joe/Leo sees that duplicity. He cannot see it with Nadine because, by then, he is too close to her.

With one or two exceptions, Stephen King doesn't write in absolutes. There are one or two characters who are clearly very good or very bad (Mother Abigail and Flagg), but on the whole his characters are more complex than that. This isn't a case of the Star Wars Sith vs Jedi, good or bad, black and white universe.

So Leo isn't afraid of Harold simply because he is from the dark side. There are several reasons why he is afraid - such Harold's physical presence, his self confidence, the thought that he might come between Leo and Nadine.

In the same way, he doesn't like Nadine because she is good or bad. He likes her because ... all sorts of reasons.

And as you will see later, Nadine and Harold aren't necessarily all bad.

Edited to answer your second question: nobody in the book knows everything. That's why some of the characters make bad judgments about other characters - even Mother Abigail and Flagg.

You think that Harold and Nadine are capable of betrayal. Others seem some good in them. And both are capable of good. King is trying to show complex real characters and not cartoony good and bad characters

I didn't mean like that.For example Trashcan Man is one of those charachters.He is not bad or good.He is just lost.I think he could end up diffrently If he came across Larry instead of The Kid,for example.For all people on both side,I have feeling that Harold and Nadine are capable of betrayl.How mother Abagail didn't had bad feeling about Harold?Will Randall Flagg have that feeling when Abagail's spies come to his camp?

I felt that Joe/Leo was seeing Harold as a person who desired the connection to Flagg and had little doubts about it. Nadine was undecided until after she moved in with Harold, and this is where we see Leo regress back to Joe for a while in judgement of her decision.

Leo came out of his regression because of Nadine. She belongs to him, as he sees it. Once she leaves him, he regresses because of her abandonment. He hates Harold and fears him because he fears losing Nadine, and is jealous of any relationship that Nadine has outside of their own. He was jealous of Larry when they met for the same reasons.