Which brings me to the real point of this post. If the single greatest motivation for a species to go interstellar, and to show up in a hostile fashion, is because their home system can no longer support them, we should be trying to find those nearby exoplanets where it looks like life is on its way out.
Being able to actually do this is a way off, so bear with me. Suppose though, in the not-so-distant-future, we do have the capability to confirm the presence of a biosphere on an exoplanet, and we have refined our techniques so that we can measure stuff like industrial pollution, climate change, and perhaps even the infra-red excess of active technology. In this case we might be able to gauge the odds that life has messed things up good and proper.