The Ass, the Fox, and the Lion

An Ass and a Fox went into partnership and sallied out to forage for food together. They hadn't gone far before they saw a Lion coming their way, at which they were both dreadfully frightened. But the Fox thought he saw a way of saving his own skin, and went boldly up to the Lion and whispered in his ear, "I'll manage that you shall get hold of the Ass without the trouble of stalking him, if you'll promise to let me go free." The Lion agreed to this, and the Fox then rejoined his companion and contrived before long to lead him by a hidden pit, which some hunter had dug as a trap for wild animals, and into which he fell. When the Lion saw that the Ass was safely caught and couldn't get away, it was to the Fox that he first turned his attention, and he soon finished him off, and then at his leisure proceeded to feast upon the Ass.

— Betray a friend, and you'll often find you have ruined yourself.