I hope this defense fails miserably. I've never consumed pot, but I know enough to know that it doesn't make people paranoid and violent. There are other drugs that can do that, but (as the article says further down), Mr. Kirk had none of those drugs in his system.
This guy planned to murder his wife for the insurance money. He went and ate a bit of pot candy to use it as an excuse. I hope the court throws the book at him. I don't think Colorado has the death penalty, but if they do, he should get it.