Why did Ryuk kill Light?

Ryuk’s decision to kill Light towards the end of Death Note was because of boredom, as he believed that having him doing the dirty work for him was no longer entertaining.

Ryuk is a fictional character from the manga series, Death Note, who is the main antagonist in the story. He is a Shinigami, which is a death reaper, that drops a death note onto earth. The notebook allows the user to kill anyone by simply knowing their name and face, and was dropped into the human world in order to relieve him from his boredom. Death Note follows the story of Light, a young genius that stumbles upon the book and is committed to using it for good and not evil.

Light’s death occurred after his name was written in the death notebook by his rivals. As Ryuk and Light had created a close bond throughout their years together, Light begged Ryuk to write the names of everyone present as a last resort, in order to save him. Ryuk, instead, wrote down Light’s name in his death note, as he had promised to do at the beginning of the story. Light then declared himself the God of the new world before dying. Ryuk ultimately revealed that his reason for killing Light was boredom, as a result of the antics he had come up with whilst in possession of the book.