Who killed Robert Baratheon?

Robert baratheon was killed by his wife Cersei. Robert was injured fatally during hunting. He was given a much stronger wile, All of this was planned by Cersei.

He was fatally injured in a hunting accident, engineered by his wife Cersei. Robert was given much stronger wine than he expected, and as he tried to kill a boar, was gored by it. Though it sounds like his death was a fluke, Varys surmised that if it had not been the boar, Robert would have died from a 'fall' from his horse, or a hunting arrow gone astray. Either way, he was doomed. He soon died from his injuries.