It's actually one of the more plausible theories. Oswald was a disturbed ne'er-do-well who seems to have been hungry for attention any way he could get it. He initially wanted to kill the ultra right wing general Edwin Walker, but failed and later just happened to be living in Dallas when JFK visited.

Narratively, though, this isn't satisfying, which is a big reason, I think, why the conspiracies continue to flourish.