The answer to the seagull soup riddle is that the man killed himself because he felt guilt and remorse, remembering a boating accident. The answer, however, is up to interpretation as the riddle's question does not give much detail about the story.
The riddle's question states: "A man goes into a restaurant and orders seagull soup. As soon as he tastes it, he runs home and kills himself. Why?" Usually, the riddle is told with a couple caveats that the soup was not rotten nor poisoned.
The question is so vague that nearly anybody could come up with an acceptable answer that fits into the parameters. The back story generally remains the same: The main person in the riddle, sometimes given a name such as Bob or Tom, used to be a fisherman. One day, his boat crashed, killing everyone on board except for himself. He ended up being stranded on an island, and the only living creatures there other than himself were the seagulls. Eventually, the bodies of the boat's crew washed onshore and he had to watch the seagulls eat the bodies. When Bob (or Tom) tasted the seagull soup, all of those memories came flooding back. He felt waves of guilt and repulsion, which led him to kill himself.