Summary

• Darth Vader likely sought revenge for offenses he endured during the Clone Wars, and Commander Fox had a challenging history with Anakin Skywalker.
• Commander Fox's fatal mistake post-Revenge of the Sith was not providing a description of Lord Vader which led to both Jocasta Nu and Vader being fired upon by clone troopers. This resulted in Fox's execution, one of the first examples showing how Vader would handle Imperial incompetence.
• Vader likely had no qualms about killing Fox due to the Clone Commander's refusal to let Anakin see Ahsoka Tano when she was in custody, giving shoot-to-kill orders once she escaped, and being responsible for the death of Fives (leading to deeper motives for Vader getting some vengeance).

One of Darth Vader's first executions within the Imperial leadership also served as some major payback from the Clone Wars. In the aftermath of Revenge of the Sith, the Dark Lord of the Sith was still adjusting to his new role as Palpatine's right hand in the Star Wars timeline. However, the Empire itself also had some initial struggles with the new Sith Lord, so much so that Vader was given the darkly perfect opportunity to get revenge for key offenses he endured in the final days of the Clone Wars when he was still Anakin Skywalker.
At the dawn of Palpatine's Empire following Order 66, Jedi Archivist Jocasta Nu secretly returned to Coruscant, hoping to restore light to the galaxy by recovering a secret list of Force-sensitive children. Unfortunately, she was ultimately intercepted by the Grand Inquisitor and several of Coruscant's security forces, most of whom were still clone troopers in the Empire's service. Darth Vader was also on the scene with instructions to bring Jocasta to his master alive. However, Imperial Commander Fox made a fatal mistake, one made even worse due to his challenging history with Anakin.

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