Too bad for Ryuk he's doomed to be bored for all eternity now. =D In the anime when Mikami takes over as Kira, Takada announces that he was going to start killing those who don't "contribute to society" (or something like that). I think Light says that it's too early to be announcing that, but it gives you a hint that he wasn't going to be done after killing all the criminals. So yes, like you said he would have gone mad. XD