I've been toying around with an idea of flipping these. Basically this:
Starts with a destruction of a world and a single ship fleeing it.Cut to Earth where some sort of superhero shows up, not really human, does some helpful stuff and all that, saves our human connection, a girl of sorts (playing on the Twilight note here) who falls in love with the dashing hero with a troubled past. He in turn continues helping the humans super heroey stuff while there's indications that something bad is coming. Eventually he reveals to his now girlfriend of sorts that he is the last of his kind, fleeing from a race of warmongering aliens who destroyed his homeworld and are bent on wiping his species out, turns to the human race for help, they agree.
Big bad shows up, big talky buildup, some wierd things are said by the big bad (our hero translates for the humans in the piece). Some clashing, something to put our girl in danger, build up tension, critical moment of the movie, hero has to save the girl but sacrifice himself or something.
He doesn't, a cold and condescending speech is given by the hero, and it is revealed that it was him in fact who destroyed the planet at the beginning, and that he is the insane sociopath who's going around and wrecking shit while the big bad is some sort of vigilante or official trying to stop his rampage. Unfortunately he fails as the hero kills him and the girl. Afterward he goes back to Earth (this happened on a ship or something) and tells that he stopped the first one, but unfortunately his ship and girl have been lost, and worse yet, there will be more of these and he will need all of humanity to unite behind him if they are to live.
Or something like that, sorry for the maybe derail, had to get that out of my head, been mulling over it way too much.