You've hit the nail on the head, and that's why American blacks are all convinced that a giant government conspiracy killed King, not a loser with a junk rifle, either.
It's what really drives most conspiracy theories; the refusal to believe terrible things can be done by insignificant people, or can occur by accident; the cause of the tragedy has to psychologically match the victim, or the effect.
Same goes for Pearl Harbor; such a disaster HAS, in the minds of the kooks, be caused by an evil conspiracy by Roosevelt, when in fact we know it was an unfortunate combination of mistakes and blindless and lack of communication within the US military establishment. However, that explanation isn't nearly as exciting as the Roosevelt explanation.
There is no defense against tinfoilhats.
I think that phenomenon also applies to "criminal masterminds". Most criminals are just plain stupid. When they accomplish some crime, in a spectacular way, it is usually only by accident and not design.