Disclaimer: I own nothing of Death Note.
When it all begun, there was nothing. Nothing but darkness.
Then there was light. Followed by angels, planets, nature, animals. And then there was the fall. Angels convinced and led by one, the greediest of them all, rebelled against their God, their maker and fell. Fell from heaven, fell from grace. All while the shinigami observed, safe from a far off distance, commenting and laughing. Entertained they were and they liked it.
After that came the first man with the first woman following. Things calmed for a while until they, too, were convinced, fooled by that same angel. They, too, were taken by greed and fell from heaven, from grace.
But their "rebellion" was nothing sort of grand like the angels'. No, theirs came by the form of a simple, innocent red apple. A shinigami's favorite food.
However for the shinigami, this "fall" was far from the simple entertainment of the last time. This was the beginning of a new era, where the shinigami would be at the top. The Persian war, the Peloponnesian, the Sicilian, the Wariing States of China, the English, American, Greek and France's civil wars, the Sino-Japanese, the World War I, the World War II, the Cold War. An era full of wars, genocides, deaths among the human nations. An era full of amusement for the gods of death.
But soon, everything lost its spark, it's fun. In the end, their job was just another obligation, another need. Wars and death became a hassle, nothing interesting, nothing worth fussing over. After all, death had become an everyday's occurrence, murder was something done without as much as a thought. Nowadays nobody did anything with a plan that could have you sitting on the edge, shivering with anticipation, begging for more. How boring.
So, in time, the shinigami also fell. They, too, were taken by their own greed when they begun craving for something even more exciting each time, making them loose what little they had. Their fall may not have been due to some rebellion or sin, however it certainly was the worst.
No longer where they the prideful creatures worshiped and feared by all. Those prideful creatures watching and killing and helping whoever they saw fit. Now with all hope for even some entertainment gone, they were reduced to lazing about all day. Wasting their lives gambling and drinking. Killing only out of need and when absolutely necessary.
There was, though, one shinigami searching and searching for something fu, for something interesting. One that had somehow maintained his thirst for thrill, unaffected by the fall of his kind. So it didn't come as a surprise when he cheated a second Death Note out of the King and dropped it in the human world.
And when after days he left his dimension and went to the human who found the Death Note, he was not discouraged to see his try end up in failure. Instead, he continued dropping it and dropping it until he found the one. One as bored as himself, with eyes that could read your soul, with a mind that could rule the entire world.. and as he heard of the boy's plans, Ryuk had one thought on his mind. How interesting.