The following contains spoilers from Star Wars: Darth Vader #31, on sale now from Marvel Comics.
As a Sith Lord, Darth Vader had plenty of enemies. The Rebel Alliance was the most obvious adversary, but there was no shortage of others. There were people inside the Empire, like Ochi of Bestoon or Sly Moore, that wanted him dead. There were also people with personal grudges that wanted the Dark Lord out of the picture. Thus, Vader was perpetually looking over his shoulder to identify his latest enemy.
Before Luke turned him back to the light side of the Force, Vader was totally ruthless. He dispatched all of his enemies without a second thought. No backstory or previous encounter could make him go soft, but there was one memory from Vader's past that would always make him angrier. This happened in Star Wars: Darth Vader #31 (by Greg Pak, Ibraim Roberson, Federico Blee, and VC's Joe Caramagna), as Vader's latest enemy, had unfortunate ties to Revenge of the Sith.
Jul Tambor Managed to Escape Darth Vader
All of Star Wars' main comic series are telling the same story. Qi'ra has spent decades building up Crimson Dawn to try and overthrow the Empire. The Hidden Empire series is telling things from her perspective, while the Darth Vader series is recounting Vader's attempts to hunt down and eliminate anyone who has had dealings with the pesky crime syndicate. Most recently, he's recruited Padme's handmaiden Sabe to his cause. The previous few issues have had two primary plotlines.
At Vader's command, Sabe has been hunting down an insurgent named Jul Tambor, while Vader has been playing cat and mouse with the rest of the Padme's handmaidens. Darth Vader #31 showed those two plots running into each other. Tambor had captured Sabe just when Vader attacked him. Tambor's kill squad nearly defeated Vader, but the other handmaidens (who were reluctantly serving Vader) arrived to save him. Tambor escaped in the skirmish which means Vader will be particularly acute on making him pay in the coming issues. The thing is, Vader already had a past with Tambor, which will make the hunt even more meaningful for the Dark Lord.
Jul Tambor Reminds Darth Vader of His Most Tragic Moment
Jul Tambor's grandfather was Wat Tambor. Wat was the leader of the Techno Union during the Clone Wars, and a newly-christened Vader murdered him and the rest of the Separatist Council at the end of Revenge of the Sith. Shortly after that, Vader went on a mission to Skako Minor, where he killed all of Wat Tambor's loyalists, along with his son, Wat Tambor II. So, everything that Jul Tambor was doing was in hopes of avenging his father and grandfather. However, Jul's identity isn't lost on Vader. The comic series has been very clear that Vader knows exactly who he Jul, and that has only awakened Vader's worst memory.
Darth Vader's worst memory was in Revenge of the Sith when he choked Padme and pushed her out of his life forever. Those actions constantly haunted him, and Darth Vader #31 actually opened with Vader envisioning a version of that scene where Padme joined him. Thus, Jul's uprising and his dealings with Padme's handmaidens, it's clear that his failures are on his mind. The thing is, Vader may not have a personal grudge against Jul Tambor, but he certainly seems to associate him with Mustafar and his actions in Revenge of the Sith. This led him to seek to eliminate Jul. Unfortunately, he failed to do so, which means that Vader will only be more determined to kill Jul in the coming issues.