Imagine a question and stream-of-consciousness explanation for which this is the answer: Cersei
Question: At the beginning of A Game of Thrones Robert, then king, has just come to Winterfell to appoint Eddard Stark the Hand of The King. He was unaware that Cersei's three children were therefore fathered by her brother Jaime. More interested in food, drink, and tourneys than in governance, Robert has squandered the royal treasury, leaving the crown heavily in debt. After he dies in a hunting accident orchestrated by Cersei, the kingdoms plunge into civil war. His death incites the war that encompasses the entirety of the following two books, the War of the Five Kings, in which his two brothers, Lords Renly and Stannis Baratheon, declare themselves the rightful kings. Robb Stark is also declared King in the North and King of the Trident, and Lord Balon Greyjoy declares himself King of the Iron Islands.

Who killed robert baratheon in game of thrones?
Stream-of-consciousness: To answer the question, consider the following: After he dies in a hunting accident orchestrated by Cersei, the kingdoms plunge into civil war.