WARNING: The following contains spoilers for Star Wars: Darth Vader #17, on sale now from Marvel Comics.
In Star Wars: Revenge of the Sith, it's a toss-up as to the most brutal execution Emperor Palpatine and Darth Vader enacted. The deaths of the Jedi younglings on Coruscant stand out, as well as Order 66 wherein several Light side users were annihilated by corrupted clones. However, one other frightening act of genocide came when Anakin, who had just turned evil, slaughtered the Separatists on Mustafar, a moment that was just paid homage in Star Wars: Darth Vader #17 (by Greg Pak, Raffaele Ienco, Alex Sinclair, and VC's Joe Caramagna).
In Anakin's act of bloodshed, as the newly-anointed Vader, he entered the control room, shut the doors down and began hacking down droids and rogues from the Trade Federation. They were used as tools by Palpatine, who had now turned the Republic into the Empire, so he needed all evidence of that sinister alliance gone.
When Nute Gunray begged for peace, Vader made it clear without a word that the deal was off, mercilessly cutting him down. Ironically, that scene was homaged earlier in this series when Vader revisited Nute's corpse, triggering memories of the murders. In this issue, he recreates the kills with Bokku and the Hutts due to a yet another deal gone sour.
When the Empire is at risk of losing Han Solo's carbonite casing, Vader heads to Bokku's vessel to attend to a deal made earlier. The Sith general was using Bokku to steal Solo from Jabba's clutches so he could have bait for Luke, but with that plan shot, Vader resorts to his one plan that never fails -- using his lightsaber to handle any potential disagreements. It's very much akin to him previously trapping and eliminating the Separatists in Revenge of the Sith.
After cutting his way through a slew of Hutts and droids, Vader gets to Bokku, who believes he has an ace up his sleeve. However, Vader quickly impales him, with the murderous act becoming the catalyst for a grander game as the ship is blown up by a dying Bokku.
This all ends up being part of the Emperor's long term plan to break Vader and get Luke Skywalker at his side. It's ironic because Vader plays into the tyrant's hands once more, although the Sith Lord suspects that Palpatine has his own plans for Luke.
When he was on Mustafar, Vader continued to be used as the Empire's pawn, killing aristocrats that could have been used to stop the fall of the Republic. This presents the compelling argument that even when Vader believes he has the upper hand, he is still little more than an instrument to be used by the Empire.