New at your local cinema is Battleship, a $300 million dollars evil-space-aliens epic loosely based on the board game of the same name. It is set in Hawaii, scene of the Pearl Harbor "cultural alien" attack by the Japanese. In this version today's Japanese and the retired battleship Missouri help save the day.
Astronomers will chuckle when the evil invaders show up quickly after we beam our presence to distant stars via multiple Hawaii-based radio telescopes, relayed and focused by a geosynchronous satellite no less. Science absurdity aside, this movie does deal with the global extinction possibility. As one of the actors said, it's like what happened after Columbus met the Indians; and this time we are the Indians.
There are multiple other dramas using the same extinction theme, going back to Orson Wells and his Martians. Each time something minor has a major impact on the outcome. Bacteria killed off the Martian invaders; and a simple Mac laptop was more recently used to defeat the Independence Day bad guys. Today's heroes used naval weapons for the same end. Each movie of this type is very patriotic, and we humans stand victorious at the end, or do we?
Mexico's Cinco de Mayo celebrates the first ever victory over invading French forces by natives. However, a larger French army appeared and defeated the locals. With nasty space aliens eager to feast on our flesh, or whatever, there is always method to their madness. These aliens seem like avatar projections of the very worst aspects of human nature, somewhat like the Mongol invasion of medieval western civilization.
Modern humans are reaching out to the stars for multiple purposes. The only place we may intend to colonize is Mars. Going interstellar is very difficult, due to the immense distances. Nevertheless, with deep hibernation, or even machine-like life, a journey at nearly luminal speeds is possible within the proximal galaxy. In other words, the bad guys must really want to hunt and hurt us to go to such efforts.
If we had unique technologies, or critical minerals, or something else urgently needed for survival in this region of the MW, maybe invade. Good and bad are in the eye of the beholder. My guess is that such "visitors" would rather negotiate a mutually beneficial arrangement, after they intimidate us. However, they might be highly enlightened and come in pure peace to help us, as did the aliens in Contact. They could even be stranded "space tourists," like ET, or the man who fell to Earth.
Looking at the list of possibilities, we cannot logically exclude the most evil scenarios. Problem is, we cannot place a probability on any scenario. Most likely, if we have already been visited by aliens, as many believe (and don't test my DNA), then our future is good, as long as a new bunch of bad guys doesn't show up to overwhelm the local alien good guys.
Good tends to overcome evil. If we use human history as a systems theory predictive model, a new hybrid good follows a bad era. Whether the initial body count is in the thousands or millions, civilization is like metal, where the best steel is tempered by the hottest flame. Ultimately, cooperation is more efficient than deadly competition.
Modern human civilization is becoming like a transitional alien society. Autonomous civilizations increase in complexity and power to where they develop the power to self-exterminate, if only by accident. I contend that the most critical event in human history was the Cuban missile crisis. We got lucky.
Visiting alien societies may have had their own version of our crisis. If all their nukes had been unleashed in a global war on a home planet, only a small population would be left in a stone age, or maybe no life forms at all would be left that we would enjoy. We might some day travel to another blue planet which is only a remnant of its past.
Any life forms coming at us could be a refugee crowd looking for a new home. More likely, they would settle much closer to their damaged home planet, on another planet within a habitable zone. Even if they only obliterate some of their already populated planets, that extinction lesson would be awesome. Therefore, I conclude that it is conceivable that anybody who shows up will not be amped on space testosterone, but full of wisdom.
Because it is impossible to exclude all Mongolian horde hypotheses, I suggest that the passive listening approach of SETI is most prudent. Sadly, their array of telescopes has been hibernating for a year due to insufficient operational funds. That's OK in the big picture. The funds will come, and SETI will again be operational: http://www.space.com/15803-jill-tarter-seti-search-retirement-qanda.html
We already have unintentionally reached out to potential aliens. Starting with early radio broadcasts, and then with early TV, we have shown the aliens who we really are, and that includes the first TV signals featuring Adolf Hitler. Hopefully, future visitors will not be in a hurry to visit, because subsequent TV featured The Honeymooners and I Love Lucy.
If I were an advanced space alien civilization looking for some land worth conquering, Earth's vast wasteland TV signals would be sufficient reason to avoid such a quarrelsome and primitive place.