"destroy democracy in Iran"
Why do people keep on saying this when it's an utter falsehood? You cannot destroy democracy where it never existed.
Could you elaborate on that? Mossadegh was nominated as Prime Minister by the Majlis, so unless you think that the Majlis wasn't elected in a proper manner he was democratically elected (women didn't have the vote but since you are not exactly the "I am a male feminist" type that is likely not what you are talking about. 
).
The exceptions to full adult suffrage don't seem so bad for the time and context: women, foreigners, under 25s, "persons notorious for mischievous opinions", convicts and ex convicts, active military personnel.
Iran was a constitutional monarchy with a Parliament and such systems do have a democratic element, which the Shah (with US assistance) blocked from increasing its power, thus curtailing a process that was heading towards parliamentarism and full democracy (or that is at least a reasonable interpretation - we will never know for sure). What happened changed a constitutional monarchy into a de facto absolute monarchy.
Iran was constitutionally where a lot of European states were in the second half of the 19th century that are generlly considered developing democratic in that era. The Shah had a habit of firing his PMs at a rapid pace, but if Mossadegh had been able to stay it looks likely they could have continued down the same grsdualist path to full democracy that constitutional monarchies took in Europe. At least the US backed "operation" blocked any possibility of that happening.
You think those were fair elections? Okay then...
Democracy is never born mature except perhaps in an amicable secession from a democratic system (Slovakia in 1993). Maybe it can form quickly when a nation secedes from a partially-democratic order and simply tosses away what was undemocratic (Czechoslovakia in 1919).
We will never know how democratic Iran would have been had Shah Reza Pahlavi II let a multi-party Majlis take shape. He chose instead to have an elective system in which only his puppets could run for and hold office, so his Majlis became a rubber-stamp. In view of how his regime ended, he made a bad choice whose consequences would not be obvious for 20 years. He could have let democracy develop slowly, but by the time the Iranian people were sick of him and his ability as a ruler failed (probably with his personal health), the most ruthless, well-organized, and conspiratorial underground figures prevailed.
Just imagine a British-style democracy in Iran... and consider some of the tragedies that do not happen. A democratic Iran might not have been able to avoid war with Saddam Hussein, but it would have avoided the life-wasting attacks upon the tyrant's armies. Democracies may not like wars, but that doesn't stop them from winning the wars that they get into. The Iranian Armed Forces deliver a smashing defeat to someone who in reality became one of the most vicious tyrants before he could become the monster that he was. Saddam Hussein might never dare gas the Kurds as he did.
