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.
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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
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Sounds a bit like "Protector" by Larry Niven.
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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 farsuperior to all other races including humans, and are forced by humans, whoinsist on attacking them while they are busy trying to save the universe fromthe dark energy photino birds, to simply and effortlessly wipe out the humanrace, though they save a small sample community of humans on an artificialworld with an artificial sun so they can continue to exist in a harmlessfashion; see Ring and Vacuum Diagrams by Stephen Baxter.
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There's The Genocides by Thomas M. Disch.
Humanity isn't even noticed in that, or The Forge of God by Greg Bear, where we get annihilated just to be on the safe side.
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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...
What Sci-Fi novel is about aliens who consider it simply convenient to exterminate the human race?
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<story-identification><novel><alien-invasion>
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Suggestions include Killing Star by Charles Pellegrino and George Zebrowski; The Screwfly Solution by James Tiptree, Jr; Protector by Larry Niven; or The Genocides  by Thomas M. Disch. Books where aliens consider it covenient to exterminate humans include Killing Star, The Screwfly Solution, Protector and The Genocides.
Books where aliens consider it covenient to exterminate humans include Killing Star, The Screwfly Solution, Protector and The Genocides.