The Prime Directive pisses me off. What defines "natural evolution?" We're all a part of nature. Just because I come from another planet and have more advanced technology than you doesn't mean I'm not natural! If a volcano threatens to wipe out all life on a planet and I have the means to stop it, not doing so would just be douchey.
Thing is, the prime directive is meant to prevent captains from playing god, messing up entire civilizations, etc. But the way it's portrayed in Star Trek, even saving a race from extinction is some form of monstrous act.
I just think the thing is ill-defined because of lazy writing.
The entire "destiny" thing is nonsense.