The hostility began in 1953 - too many Iranians hated shah and seen monarchy as oppressors put in by CIA. Even anti islamist Iranians were writing about it. And yes, monarchy was a vialent dictatorship.

Like yes, goverment put in by america wont be hostile to america. But its opponents will blame america for that goverment, because well, they are to blamr. And also, once it fails, all the suppressed anger goes out. And some more, because now america is well positioned to be generic scapegoat even for stuff it had not done.