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'.