I see only two possible reasons why we would be met with hostility by Alien beings.

1.) They are extremely Empirical. Devoted and driven by some sort of religious or elitist zeal with an everlasting need for conquest. Their sole purpose is to conquer civilized planets and conform the lesser lifeforms to their dogma (Think Magellan and the Philippines). Sagan stated that for any civilization to be able to traverse the stars they would have to outgrow their hostilities and forget the concept of war. Could the opposite have the same effect? Could there be a civilization in which war is everything to them, and their unquenchable thirst for conquest has driven them on a crusade across the stars? Who knows... consider all of the warrior societies and Empires that have existed on earth: Mongols, Macedonians, Spartans, Romans, Vikings, Saxons, Huns, Samurai, Sioux, ect.

2.) A hive-like armada of machines controlled by a single Hyper-Intelligent AI. It would have recursive self-improvement. Which would cause it to constantly be better at improving itself, and would continue doing so in a rapidly increasing cycle, leading to the emergence of an intelligence that would not have the limitations of human intellect, and pretty much be able to invent or discover almost anything. I imagine it would have destroyed its creators and would probably see species like us as its greatest threat. It would then lead an army of drone-like machines, all controlled simultaneously by its Hyper-Intelligent-Hive-Mind on a quest to cleanse the galaxy of anything that could threaten its existence. Such an enemy would be theoretically immortal, completely immoral and probably have spent its time exterminating countless sapient civilizations over millions of years.

All in all a war against any alien species would last a matter of minutes. It would be like chimpanzees fighting the US airforce.