The Iranian government's cruelty doesn't have to be exceptional. The question has already been asked; it is still either true, or false. The claims by the U.S. about the Iranian government stifling free speech and arresting and executing dissidents: are they lies, or not?
Side issue: if it's morally right to beat the crap out of one oppressive dictatorship, that doesn't mean we're morally required to beat the crap out of
all of them. The current government of Iran should be destroyed; so should the government of China, and the government of North Korea, and the government of Saudi Arabia, and several others I probably don't need to mention because I'm pretty sure you get the idea.
It's morally right to destroy all these dictatorships; it's simply not possible for the U.S. to do it
because we don't have enough troops. Cue cheesy analogy: if it's morally right to donate free housing for homeless people, does that mean we shouldn't donate at all unless we can donate for
all of them.....? I'm guessing your answer to that would be "no, we should donate as much as we can afford". And there you have it.
Other side issue: I never mentioned anything about nation-building. I'd be fine with bombing the current Iranian government out of existence and then just strolling home and watching a hockey game. Hell, nobody else helped the U.S. nation-build two centuries ago when we kicked the British back across the Atlantic. Eliminating oppressive regimes is still a vast improvement, nation-building or no.