"Darthipedia bores me. Watching humans type unfunny nonsense about Star Wars doesn't satisfy me. I'm gonna write your planet's name in my Darth Note to spice things up a little, watch out <insert name here>..."
"Misa no liken him because he wanna kill Misa boyfriend! Not only that but hesa rape Misa all the time and issen bewwy muy muy hurty! Hesa bad! HESA BEWWY BOMBAD!"
Eventually, because Ryuk was bored yet again, he decided to kill Kira by writing his name in the Darth Note. What a jerk.
Other than this, Ryuk was known to participate in other activities to keep himself interested (and fail doing so), such as feeding his insane apple addiction and raping(understatement) Darth Misa-Misa. It is rumored that he went to high school with Darth Animal, but this has yet to be confirmed.