I'll grant that the "idiot" part is less damning, for exactly the reasons you mention -- maybe it's possible that he was young and naïve, and maybe it was possible that he could overlook the potentially dramatic consequences of signing that contract. But honestly? If you're precocious enough to be taking investments from strangers to build a business, you're precocious enough to read and understand a simple contract. I'm not inclined to give much sympathy on the basis of ignorance.
As for the sleazebag count, I'm not sympathetic at all. If it turns out that Zuckerberg did sign the contract in good faith, and that he did subsequently ignore it when Facebook blew up, that's pretty crooked. Maybe you can argue that he simply forgot about the contract, but that goes back to idiocy -- wouldn't you remember the guy who gave you $1,000 to buy a 50% stake in your company?
No, something is fishy here, and if the contract is real and not signed under duress it doesn't speak well to his judgment. Considering the history of claims against him, I'm not willing to assume that Zuckerberg is a boy business genius while simultaneously forgiving huge legal gaffes like this one.