Hmm. I'd hope that the aliens would have realized the mistakes associated with genocide, just as humanity, for the most part, already has. Still, being cautious is never a bad thing. In all likelihood, I think that we're more likely the discover a less-intelligent species than the other way around, given what we know about the universe already. I mean, since we have not met alien life yet and are in the process of developing more advanced space travel technology, if things go on as they have been, we will be fully prepared to explore space and still not have met alien life in a few centuries. In that case, we'll have to be the instigators and establish first contact ourselves.