Hope me, science fiction readers! I can't remember the titles or authors of two short stories:→[More:]
One is about aliens who come to earth and announce to us that we are an experiment of theirs gone terribly awry. Having decided that we are their greatest creation and worst failure, simply beyond rescue, the only solution is to destroy us as a species.

We gather all of the world leaders and implore them for a second chance. At length, they agree and agree to return in the not too distant future to review our progress.

For the first time in known history, we manage to create a structure for peaceful and harmonious co-existence. With no small amount of pride and relief, when the aliens return, we show them our achievements. Expressing even more disappointment, they inform us that we were created to be a species of blood thirsty war 'machines' beyond compare and have now proven how complete their failure was...and, of course, destroy us.
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The second one, I could have sworn was written by Charles Sheffield, but have never been able to track it back down. Its premise is much more basic [and I think it's told in the form of a scientific interview regarding a current fictional situation]: that similar to the 17 year locust, dinosaurs operate on a millennial [?] cycle. And, yeah, they're back.