He cautions that aliens may become hostile if we reached the ability to launch Star Trek-style space battle cruisers. "Any spacecraft capable of interstellar voyages in reasonable time is by itself a weapon of mass destruction," he writes. Even worse, a starship would show up without warning. The infrared signature from an alien vehicle heading towards the Earth at ten percent the speed of light might be detectable no farther than 2 billion miles from Earth. We'd only have 12 to 24 hours of warning before arrival (or impact!).
ANALYSIS: Do Aliens Exist? If So, Will They Kill Us?
There is a long list of arguments for aggressive societies: eagerness to consume our resources, a belligerent ideology, a desire to be the sole galactic power, indifference to our existence, or a combination of the above. What's more, some alien societies might just be pathologically xenophobic.
My best guess is that they could have purely unfathomable motives behind our elimination.
Korhonena says that a scenario not widely talked about is an alien first strike aimed at eliminating competitors. This buries deep in the Darwinian psyche. A lesson learned from the Cold War is that two fundamentally antagonistic civilizations with the capability of destroying each other had a rationale for launching a preemptive first strike.