Eminent scientist Stephen Hawking has expressed concerns that extraterrestrial life may not be what we assume. Aliens may be dangerous rather than charitable and giving, he asserts in an upcoming Discovery Chanel special. If nothing else, cultures advanced enough to reach Earth may have an effect similar to the discovery by Europeans of the Americas: devastating to the so discovered.
Other scientists disagree: many believe a sophisticated alien race reaching our world could only be good.
Truth be told, we really don't know what to expect. Who's to say aliens would be bad? Or good? It's all simply presumption. That said, the only honest appraisal of contact with any given extraterrestrial life form would be the same as with meeting any given stranger on Earth: approach with caution, olive branch extended yet with the proper amount of reserve to counter their actions and intentions if need be.
Our science fiction has had a great propensity to view potential alien life as superior to ours in almost every way. It is good to see a prominent scientist point out that that may not be the case.
In the end it may all be conjecture. Presuming that other lives on other planets would face the same daunting tasks of physics as we do, interstellar travel itself may be little more than science fiction. The folks at SETI, the Search for Extraterrestrial Intelligence, seem to think that the best we can hope for is hearing them through radio transmissions. That sounds like the most reasonable prediction on the matter.
In short, the possibility of sentient life other than us in the universe is fun to discuss but nothing is likely to come of it. Still, it is good that Professor Hawking has sought to balance the scales. There's no good assuming that technical superiority means morally advancement.