Point out that we have never destroyed entire sentient species, like they are implying. Evolution is a logical fact of any sort of system of life, and Evolution instills very violent realities into an individual. Assuming multiple races of alien life exist in the galaxy, odds are we are not an outlier in the viciousness bell curve. Our rather hospitable planet means that odds are, we are a relatively peaceful, kind and benevolent sapient species. And of course, distance means that really, we are no threat to anyone outside of our solar system. If they can travel to us and threaten us, then they will always have the upper hand, and the distance also means that any sort of violent action between the species would be completely meaningless: A successful species would not bend massive resources to destroy a distant race just for shits and giggles. People are too selfish and interested in material gain for that. Realistically, the only exchange that would occur between the species is exchange of information, which can only benefit both parties. Extermination is illogical.