There is an unmistakable logic pattern in conspiracy theory.
The sane and normal pattern in evaluating evidence is to speculate one level, but then test that first speculation for veracity before building a second-order speculation on top of it. The conspiracy thinker is simply someone who can’t think clearly enough to distinguish between his own first-order speculation and established facts, so he piles speculation on top of speculation, thinking they are summing up cumulatively — not remembering that 10 times zero is still zero. When the crank recites 10,000 speculations, he thinks he has just pointed to the number 10,000 instead of the number 0.
Your main conclusion from reading something like The Davinci Code should be that Lee Harvey Oswald shot Kennedy, acting alone.