Why did Nick Andros sleep with Julie Lawry in Stephen King's 'The Stand'?

Answer 1

That can only be said to be because the author wrote it that way. It certainly was not in character. The character as wrote would not have left a retarded man alone outside in fear, just so as to have sex with a stranger. The mini-series movie did well to change that.

Answer 2

Nick Andros sleeps with Julie Lawry in a moment of weakness and vulnerability. Julie manipulates Nick's emotions and takes advantage of his isolation to seduce him. It highlights the complexities of human relationships in the post-apocalyptic world of the novel.