Det här är troligt " The Monsters ", av Robert Sheckley, ledarhistoria i samlingen " Worlds Store (det är där många människor stöter på det).
'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!"