The ending of 'Death Note' is quite complex. Essentially, Light, the main character who had been using the Death Note in his self - perceived quest for justice, is finally defeated. His downfall shows that his extreme methods of creating a 'new world' through killing those he deemed as criminals were ultimately wrong. It's a cautionary tale about the abuse of power and the distortion of justice.
In fact, the manga of Death Note has its unique ending that sets it apart. It's a conclusion crafted specifically for the original comic format, with elements and developments not found in other versions.
At the end of 'Death Note', Light's character deviated far from his initial self. He began as a student with a strong sense of justice, but the Death Note's power made him believe he could be a god - like figure dictating who should live and die. Misa, who was initially in love with Light and a part of his plans, was left broken in the end. L had a consistent development of being a brilliant mind in the hunt for Kira. His successors, Near and Mello, also grew in their own ways in the fight against Light, with Near finally outsmarting him in the end.
The ending of 'The Birds' is both mysterious and thought - provoking. After the intense and harrowing experiences of being attacked by the birds, Melanie, Mitch, and the others are able to get into a car and start to drive away. The birds that were once so aggressive have become quiet. Hitchcock deliberately left this ending open - ended. It could be that the birds' aggression was a short - lived phenomenon, or it could be that they were simply following some instinctual pattern that had run its course. This ending also serves to heighten the sense of dread and the unknown. We are left with the characters, and by extension, the audience, still in a state of fear and uncertainty as they leave the scene of the attacks, not knowing if the birds could resume their attacks at any time in the future.
Ryuk would probably just be amused. He is a Shinigami who has seen a lot in his existence. Fanfiction is just another strange human creation to him. He might chuckle at some of the more outlandish ideas in the fanfictions, like a character having superpowers beyond the Death Note.