The whole Kira thing was nothing but amusement for Ryuk. Something that would ease his boredom. When Near (and to an extent - Mello) defeated Light, the "fun" was over.
Ryuk also brought up a rule regarding the Death Note stating that the Shinigami is obligated to kill the person who picked up the Death Note 由 写作 their name in the notebook when they die. He reminds Light about this rule as Light is practically begging for his life.
Ryuk specifically says the following; "I told 你 in the very beginning that I would be the one 写作 your name in the notebook when 你 die. That is the rule between the Shinigami who brings the notebook into the human realm and the first human who picks up the notebook. If they put 你 in prison, who knows when 你 die? I don't want to just lie around waiting for 你 to die, so it's all over. 你 should die right here" (Chapter 107)
