Original Sin ends with several big changes in the Marvel Universe:
Thor is no longer worthy to lift his hammer Mjölnir because of a secret Nick Fury whispered to Thor;
The Orb shot Uatu the Watcher in the head and cut out one eye;
Nick Fury fired the final shot that killed Uatu the Watcher;
Uatu the Watcher threatened Fury on purpose to get Fury to fire the fatal shot;
Fury took the Watcher’s remaining eye;
The Winter Soldier took Fury’s place as the “man on the wall”;
The Watchers punished Fury by making him a chained “watcher” on the Moon, unable to take any action.
Here is my take on Nick Fury: Everything he did was right. Fury saw the world in terms of Realpolitik, which understands that protecting a nation (or in the Marvel Universe, the entire Earth), requires someone who operates in the shadows to eliminate the threats that would destroy freedom. There cannot be any detente with a foreign power, whether it was from another dimension or planet, that sought the Earth’s destruction. Fury’s actions kept interplanetary wars from erupting (and when one did with the Skrulls, Fury was prepared).
The problem with Fury’s actions is that private individuals are not supposed to have their own foreign policies. The Logan Act (notice, not Wolverine Act), prohibits US citizens from directly or indirectly commencing correspondence with a foreign government with the “intent to influence the measures or conduct of any foreign government or of any officer or agent thereof, in relation to any disputes or controversies with the United States, or to defeat the measures of the United States…” 18 USCS § 953.
Fury was not sending correspondence with inter-dimensional being, but Gamma bullets to eliminate active threats to planetary security. Fury’s actions would be justified if the United States was declaring it was defending itself, thus ordering strikes on threats, but Fury apparently was writing his own orders on who to kill (unless there is some Kennedy-Era Executive Order giving Fury orders to defend the planet).
Were Nick Fury’s actions justified from a foreign policy perspective? Yes. The logic is difficult to refute, which is why the Winter Soldier took up Fury’s station as the man on the wall. However, this appears to be without any legal authority, even though it is the realistic solution to actual planetary threats operating in the shadows.
As for killing the Watcher, Fury had a solid self-defense argument that the other Watchers ignored: Uatu the Watcher raised his hands charged with energy to threaten Fury. The Watcher’s power easily could have killed Fury. As such, Fury was legally justified to shoot Uatu the Watcher.
However, the Watchers permitting the Orb to escape while Fury was chained to the Moon for the rest of time, forced to watch without interference, appears to be an extremely flawed legal system. Letting the Orb, who committed attempt murder and torture by mutilating Uatu, run free with one of the Watcher’s eyes in his chest is not a proportional punishment. If anything, the Orb now has depth perception and possibly binocular vision, which would be personal enhancements.