I don't remember the title/author of a Sci-Fi novel, where the author demonstrated that in case of an encounter with a more advanced alien life form, they would consider most convenient for them to exterminate the human race instead of every other alternative...
I think this might be Killing Star by Charles Pellegrino and George Zebrowski. After aliens destroy the Earth with relativistic bombs they explain to the last human that because all civilizations are potentially dangerous, and there's really nothing to lose by exterminating them before they get too strong, it was the logical choice.
Reminds me of The Screwfly Solution by James Tiptree, Jr - a short story in Warm Worlds and Otherwise. http://en.wikipedia.org/wiki/The_Screwfly_Solution
In Stephen Baxter's Xeelee sequence, the Xeelee are fighting a dark energy enemy far superior beyond human comprehension and are simply annoyed that the humans keep pestering them with attacks in an attempt to bring "peace" to the universe by subjugating all other races. The Xeelee are far superior to all other races including humans, and are forced by humans, who insist on attacking them while they are busy trying to save the universe from the dark energy photino birds, to simply and effortlessly wipe out the human race, though they save a small sample community of humans on an artificial world with an artificial sun so they can continue to exist in a harmless fashion; see Ring and Vacuum Diagrams by Stephen Baxter.