His biggest mistake, besides taking L's bait for whore-attention, is that he killed criminals with heart attacks instead of making them kill each other in gang wars, besides being less suspicious, it would divert more media attention from Kira, although perhaps that was his initial megalomaniac goal from the beginning. In any case, he was already a God with the Death Note before he murdered people because he not only had the power to choose how they would die, but what they could do before that and how long it would last. He literally had the power of Fate/Causality and if he wrote his own name in the Death Note telling the world to bow down to him in exchange for him choosing the day he would die, so it would be.