Anakin Skywalker's turn to the dark side in Star Wars is depressing enough, but it becomes even more so when looked at from R2-D2's perspective. The white and blue astromech is one of Star Wars' longest-tenured entities, appearing in nearly every film and TV series alongside his best friend C-3PO. He has seen it all, with his memory banks dating back decades before the end of the Skywalker saga. He even witnessed his master's transformation into the feared Sith Lord, Darth Vader, firsthand. Anakin's tale is a tragedy as it is, but a different, more intimate point of view amplifies this effect.
The bond between The Chosen One and R2-D2 goes back to as far as the beginning of the prequel trilogy. They first met aboard Queen Amidala's starfighter during Star Wars: Episode I - The Phantom Menace. Anakin was still very young, so he grew up and became a Jedi with R2 at his side. Star Wars: The Clone Wars detailed their friendship at length, devoting entire arcs to it. They were inseparable, even as Anakin's faith in the Jedi Order and the Republic started to falter. R2 had no choice but to follow his owner into his dark descent into the Sith ways.
Star Wars: Episode III - Revenge of the Sith is the film in which Anakin's long-awaited downfall comes full circle. It depicts many pivotal moments that finally send him over the edge. One of the most heart-wrenching examples is when Anakin traveled to the impractical lava planet Mustafar to assassinate the Separatist leaders. He went in himself to do the deed, requesting that R2 stay with the ship until he returned. Afterward, the poor droid witnessed Anakin choke his wife Padme and then engage in a duel with his former mentor, Obi-Wan Kenobi. He was present for the birth of Darth Vader, but also the death of his former Jedi companion.
Senator Bail Organa takes ownership of R2-D2 after Anakin completes his metamorphosis into a Sith Lord. By proxy, he is the enemy of the Empire, as Organa is a staunch supporter of the Rebellion. R2 is left to believe that his friend did not survive his duel with Obi-Wan on Mustafar without the knowledge of Vader's true identity. He is committed to serving the Rebel Alliance, although the impression of Anakin's dark journey still lingers in his archives. For decades, he relived those images — unlike C-3PO, who only recently regained his memories of that time.
While Darth Maul is considered one of Star Wars' most tragic characters, there is a case to be made for R2-D2 as well. His journey isn't inherently sad, but the events he saw are of the most traumatizing variety. He watched his friend and companion become a tyrant, establishing an Empire rooted in the dark side. He wasn't even afforded the luxury of a memory wipe, trapped reliving painful moments for the duration of his life. Star Wars went the extra mile by making Anakin and R2 so close, which served only to make his fall to the Sith harder to swallow when taken from the astromech's position.