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This question already has an answer here:

I got most of it in the title, but here you go anyway:

I think it's only revealed part way through that they're aliens, and the creatures that land on their planet are actually the human ones. The humans give the aliens potatoes or something. For some reason one of the aliens wonders if one of the humans wants him to kill his wife, so he does so. The guy starts shooting them when earlier he said he wouldn't hurt them, and the aliens are shocked that he 'told an untruth'.

Cheers!

marked as duplicate by Jason Baker, Ward, Rand al'Thor, Firebat, Politank-Z Oct 23 '15 at 17:50

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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This is likely "The Monsters", by Robert Sheckley, lead story in the collection "Store of the Worlds" (it's where a lot of people run into it).

'Shall we go down and have a closer look?’ Hum asked.
 ‘All right. I think we have time — wait! What day is this?’
Hum calculated silently, then said, ‘The fifth day of Luggat.’
'Damn,’ Cordovir said. ‘I have to go home and kill my wife.’
'It’s a few hours before sunset,’ Hum said. ‘I think you have time to do both.’

....

"Now will you believe me!" Cordovir shouted. "The creatures told a deliberate untruth! They said they wouldn't molest us and then they proceed to kill seventeen of us! Not only an amoral act — but a concerted death effort!"

  • If you can click on Accept (Green checkmark), we can close this as a duplicate. :) – FuzzyBoots Oct 22 '15 at 16:31

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