Sometimes Light regrets winning how he did.
Had using a shinigami even counted as fair game? L hadn't been able to predict that, he hadn't even been able to counter it. Light had more or less played a game against Rem, in which L and Watari had been pawns lost in the battle of white and black, good and evil.
In which Rem and L were evil, and himself most certainly good.
For Kira was good, right? Criminals had done bad things with their lives; they were the enemies. They did not deserve to live amongst the unscathed and pure. The tainted criminals had no part of his perfect world.
L had been an enemy that needed elimination. But after he was gone... Then what?
The ride was almost too easy. It was a smooth form of comfort that seemed too stale and different to be enjoyable. Where was the stress of the moments, the constant threat of being found out? Where was the excitement he needed to keep himself occupied while new criminals rose to the chessboard for him to slice down like the pawns they were in creating Kira's new world, Kira's chessboard ruled only by queens and himself. An impenetrable fortress with soldiers to move in every direction, all protecting their king, their God – Kira.
There was a limpness in his life after L had passed. One day, he would be seen as God. One day, he would be able to proclaim to the world who he was and feel no need to hide behind the notebook. One day there would be no need for Ryuk's gift, the fear and admiration so deepset in his people that they would bring criminals to his feet and have them personally executed to prove his power.
One day that would all happen.
But for now, in that moment, he had only the notebook and the names of the targets.
He did not have the fear of being caught.
He did not have a challenge.
He did not have an opponent.
Sometimes, Light regrets killing L, if only out of selfish need.