Star Wars: 10 Things You Didn't Know About R2-D2
6. R2-D2 Briefly Served Darth Vader
In spite of his independent (and occasionally rebellious) nature, R2 is a loyal droid, serving his owners well and often going above and beyond what's expected of a droid. Particularly during The Clone Wars, he showed his qualities as Anakin's astromech, taking part in hundreds of battles alongside the Jedi Knight and helping him out of tight spots more than once. While this loyalty is commendable, it eventually led to him serving a much darker master.
Immediately after the death of Mace Windu, Palpatine made Anakin his apprentice, giving him the title of Darth Vader. His first command to his new student was to travel to Mustafar and kill the Separatist Council hiding there, bringing The Clone Wars to an end. Vader flew to the lava planet in his Jedi Starfighter, taking R2 with him.
Either the little droid was unaware of Anakin's new nature, or else his programming meant he still had to follow his orders, but he didn't seem to question him. It would have been interesting to see if he continued to serve Vader during the days of the Empire, and whether or not he would have participated in the Sith Lord's horrific acts.