Your right that freedom and democracy has never been anything but a byword for American Imperialism in order to appease the public. Hell, in the 1800's and early 1900's the US didn't even make a secret of its Imperialism (Philippines for example). It was only when it realised that it couldn't compete with its European counter-parts that it started taking a more subtle approach and started going on about spreading freedom and democracy.
As for whether its a force for good or bad- that's far too simplistic. In my opinion some actions have been for good and others for bad. I just view it as the way of the world. At the moment it is the USA's time as the worlds chief power and, like all the major powers before it and probably all the major powers after it, its going to look after its own interests at the expense of others and that is going to undoubtedly involved

ing over other countries or, if need be, supporting brutal tyrants and whatever else comes with imperialism/neo-imperialism and the quest for dominance. If the USA wasn't doing it, someone else would be (Indeed other nations already do).
Freedom and democracy can be a very welcome side effect of US imperialism, but its just that, a side effect. And on the other hand, if need be, the US has shown its more then willing to support brutal regimes that hold values that are the complete opposite of freedom and democracy (Saddam Hussein, Pol Pot etc)