Stephen Hawking tells us that we should probably not try to contact aliens.
He suggests that aliens might simply raid Earth for its resources and then move on: “We only have to look at ourselves to see how intelligent life might develop into something we wouldn’t want to meet. I imagine they might exist in massive ships, having used up all the resources from their home planet. Such advanced aliens would perhaps become nomads, looking to conquer and colonise whatever planets they can reach.”
He concludes that trying to make contact with alien races is “a little too risky”. He said: “If aliens ever visit us, I think the outcome would be much as when Christopher Columbus first landed in America, which didn’t turn out very well for the Native Americans.”
The idea that extra-terrestrials could be scary isn't exactly new idea, the alien invasion trope has been used again and again in fiction ever since H.G. Wells' War of the Worlds.
I would somehow prefer to believe that whatever aliens we came across would have developed morally to a point where they think just kill us, but this obviously isn't necessarily the case.
Among the greatest defining features of humanity is our rapacity. Whenever early humans arrived in a new part of the globe, most of the other life forms died off soon after. It could be that the aliens operate in much the same way. What if the aliens are simply rapacious, and don't care about any other forms of life they come across. Daniel Drezner on comments:
Why would aliens go after the inhabited planets? Ceteris paribus, I'm assuming that aliens would prefer to strip-mine an uninhabited planet abundant with natural resources than an inhabited one. Three hundred planets have already been discovered in the Milky Way, and there are "likely many billions." Even rapacious aliens might try some of them first before looking at Earth, since we are mostly harmless.
There is a counterargument, of course. Over at Hit & Run, Tim Cavanaugh tries to assuage fears of aliens by observing, "Why would a race of superintelligent jellyfish or blue whales even take notice of us, let alone want to conquer us?" This cuts both ways, however. If those jellyfish fail to notice us but notice our abundant amounts of salinated water, they could decide to come without a care in the world for the bipedal inhabitants of Earth.
One can imagine a callous species of aliens, who are as uncaring toward human interests as most of humanity is to the inhabitants of the forests we chop down. Think of the Vogons from The Hitchhiker's Guide to the Galaxy, who demolish Earth in order to build a hyper-space highway, or these aliens from Calvin's imaginings in Calvin and Hobbes (click the pictures for more legible view.)