Riddle: Once upon a time, John was walking in the woods to go fishing for some food but got lost. But he got across and old house and asked the owner, the owner invited John and they began to tell a story. The old man said "Once upon a time, two best friends were on a deserted island. There was no food at all, so they began to die of hunger. They agreed to fight until one of them died. The winner would eat the loser and thus will be saved. Besides, no one would ever know about that. They began to fight. And finally one of the best friends won." Meaning that the other was eaten and betrayed. Then, John pulled out a gun and killed the old man at once. Why would he do it?