Since nobody seems to know where the quote is from, I'll tell you. It's from the movie
The Day The Earth Stood Still. It was originally released in 1951, but the quote is from the 2008 remake. The following conversation takes place between Klaatu (Keanu Reeves) and Jennifer Connelly (Helen Benson):
Helen Benson: You said you came to save us.
Klaatu: I said I came to save the Earth.
Helen Benson: You came to save the Earth... from us. You came to save the Earth *from* us.
Klaatu: We can't risk the survival of this planet for the sake of one species.
Helen Benson: What are you saying?
Klaatu: If the Earth dies, you die. If you die, the Earth survives. There are only a handful of planets in the cosmos that are capable of supporting complex life...
Helen Benson: You can't do this.
Klaatu: ...this one can't be allowed to perish.
Helen Benson: We can change. We can still turn things around.
Klaatu: We've watched, we've waited and hoped that you *would* change.
Helen Benson: Please...
Klaatu: It's reached the tipping point. We have to act.
Helen Benson: Please..
Klaatu: We'll undo the damage you've done and give the Earth a chance to begin again.
Helen Benson: Don't do this. Please, we can change. We can change.
Klaatu: The decision is made. The process has begun.
Helen Benson: Oh God.
The TOG