The Dark Light Years
Brian W. Aldiss1964

The human species has begun to racket about the galaxy. When they reach the planet Grudgrodd, they come across another space-faring species. It's a case of instant dislike. The gentle Utods do not feel pain, they change sex as the planet changes suns, they live long pleasurable lives, free of stress. They wallow in their own middens. But 'Civilisation is the distance man has placed between himself and his excreta'. So the carnage begins.