Yes, all kind of bad things could happen if they saved them. But all of that is and can only be speculation. Their death is not, it’s inevitable. So again, I think it doesn't make sense to weigh these two things against each other. And I would argue a moral society has the obligation to help someone who’s about to die but can be saved.
I say, even if they somehow knew with absolute certainty that a planet they can save and is about to be destroyed will later be their ardent enemy, they have the moral obligation to save them if they can. In my view that’s the responsibility that comes with being in possession of their advanced technological possibilities.
What about them? Not sure if that’s what you’re trying to say, but planet-wide destruction and extinction can never be considered “societal development”. However bad you end up influencing their “natural development” while saving them, surely it could never ever be worse that their, you know, death.