One of the odd points in the movie was that he passed the lie detector test. This brings up several points:
1) Since we know he was guilty, why would he agree so readily to take the test?
2) How did he pass it so easily?
3) Why would the DA even bring a case against someone who passed a polygraph? Even though polygraphs aren't admissible as evidence, prosecutors often won't bring a case against someone who passed the test so easily.
He's an ice man remember.
Ooooh, you wicked, wicked monkey! Robin Williams in Mrs. Doubtfire
If one doesn't feel guilt or remorse for something they have done is wrong like murder the needle won't jump and there are people that can lie and pass it because they are just good at lying.
Y'know, I could eat a peach for hours
It has nothing to do with consciousness of guilt - it's consciousness of lying. The only reliable way to beat a lie detector is to tell the truth or what you believe is the truth. If he is asked "Did you kill your wife" and he says "No" and the machine says he is telling the truth, that would mean he really believes it. But nothing in the movie suggested that he had put the memory out of his mind or no longer knew he was guilty.
And yes, the polygraph isn't perfect and sometimes people can beat it, but there is no way he would know in advance that he'd be able to beat it. Experienced polygraph operators know all the tricks people use (such as holding your breath or clenching your anus or whatever) and can usually tell when a subject is trying to defeat the test. It just struck me as a plot contrivance that he was so willing to take the test and passed it so easily. I think the movie might have been more effective and realistic if the operator had reported the results as inconclusive.