+Mr.Pringels Well, we can say that Light thought Ryuk was on "his side" because he has helped him during the early periods of the anime/manga. This contradicts to what he says, "Didn't I say in the beginning... when you die; the person who will write your name down will be me" (Ep. 37 of the Anime) In addition, L and Light are both childish. They hate losing. Take that last sentence into account when Light loses.