we want to believe the elaborate conspiracy because we can't accept the banality of evil. Surely there would be a 'better' reason to shoot hundreds of people at a concert than insanity or some unstated grudge, right? Sorry, but people are capable of nightmarish acts for just about any reason, or no reason at all.
I agree with the rest of your comment, but I've never been convinced of the above as the reason for people believing in (implausible) conspiracy theories. I think there are other things going on, psychologically, culturally, and politically. Not that I can quite work out what it is. Probably multiple things, for different people and different conspiracy theories.
After all, a tendency to grasp at conspiracy theories is not evenly spread among all populations (and many of the groups who are more prone to it are also those who have a long history of being conspired against).
It's also obviously self-flattering to believe you alone are uniquely insightful and able to see the flaws in the 'story we are told', unlike all the sheep, so it appeals to those who need a source of self-esteem.
And there's also something in them that discounts large social forces and tendencies, in favour of groups of individuals determining historical events in an autonomous way, implying that at least some people (the conspirators) are completely 'free' to act according to pure will. There seems to be a powerful belief in individualism in there somewhere.
(From introspection) I'd say it's also about anxiety. Conspiracy theories might be a sort of external equivalent of hypochondria.
And of course a large proportion of such theories end up blaming 'the Jews' for everything, so sometimes I suppose it's just plain old anti-Semitism.
I guess for me it's not conspiracies vs random evil, so much as 'conspiracies' vs determinism and deep social forces that no individual or small group can control. I did at one point take seriously the JFK conspiracy theories (damn you Oliver Stone!), but I no longer give them any credence, and ultimately the main reason why I wasn't that invested in them was that I don't believe JFK's assassination actually changed history very much.