I'm not so certain that Gaddafi would have been killed if Libya had been a nuclear state. The US-backed insurrection was enabled in part because of the lack of nuclear threat from his regime - where are the color revolutions in Russia or North Korea, for example? In the end it's a historical "what-if".

But yeah backing out on the Iran deal was definitely the final nail in the US soft power coffin. Libya was betrayed by us secretly over a decade later, Iran was done about two years later completely in the open, and for a second course we assassinated one of their top military guys while he was acting in a diplomatic role in another country.