Cowboys loved a colorful phrase! This means the real thing. If it's Simon pure, it's pure.

Many scholars and reviewers have described Simon as a 'Christ like' figure.

Simon Pure

see http://en.wikipedia.org/wiki/A_Bold_Stroke_for_a_Wife

Simon was a pure, good-hearted character who just wanted to help others. He was brave, and passionate. He was the "Christ Figure" in Lord of the Flies. But he was murdered.

Having Simon beaten to death after being mistakenly identified as the beast was a moment of pure irony in Lord of the Flies.