There are too damn many humans on the planet anyway.
Seriously though, I think we have to get to the aliens before they get to us. Judging by the history of different human cultures, it always always sucks to be the less advanced people during a period of major cultural clash. If we can just manage interstellar flight, and get some exploring done before stumbling on less advanced life on other worlds, I think we might be alright.
How would we destroy them if they're like a billion years ahead of us though. They'd probably just giggle at us.
More to the point if they have interstellar travel via FTL, the types of weapons they would have would probably annihilate entire solar systems. If they even suspect we're a future threat, we wouldn't exist right now.
Also imagine a civilization that uses wormholes to slowly tear apart a planet and deliver the fragments to a desired location for mineral processing. Hell we may never even see a single ship let alone a single being before it's too late if they're that advanced.
