He knew Obi-Wan had dropped his defenses. That along with him disappearing made him feel as though he was robbed of his victory and would never get the chance to defeat him. Short version it pissed Vader off.
Anakin Skywalker ultimately ended up hating Obi-Wan Kenobi for many misguided reasons, but Darth Vader's subconscious response to the Jedi's final declaration of love in the Star Wars prequels shows the real reason he could never forgive him.
This resentment culminated in the epic climax of the entire prequel trilogy, with Anakin and Obi-Wan's fateful battle on Mustafar in Episode III. Then, Vader's hatred for Obi-Wan was solidified before their final encounter in Episode IV during the Disney+ series Obi-Wan Kenobi.
DARTH VADER FINALLY FORGIVES OBI-WAN KENOBI(CANON) - Star Wars Comics Explained
Why did Darth Vader hate Obi-Wan so much?
Having been completely and utterly defeated during their first battle in Revenge of the Sith, Darth Vader harbored a grudge against his former master which fueled his hatred for many years. Obi-Wan Kenobi's third episode confirms Darth Vader's biggest mistake came when he had a chance to finish the job.
I did ... the same way I will destroy you!" At that moment, Obi-Wan was right to believe his former apprentice was "truly dead" as Vader himself said that he killed off Anakin and any part of the latter that still existed within him. There was no going back to the light side.
After his redemption, after bringing back balance to the force, after the return of the Jedi, Anakin would have definitely held a bit of remorse, not to forget, it was Obi-wan who taught Anakin how to become a force ghost, to stray oneself from the cosmic force and assume identity with the living force.
The canonical comic Star Wars: Darth Vader #7 confirms the younglings' deaths caused Anakin grief, self-loathing, hatred, and pain, fueling his descent even further into the Dark Side.
Obi-Wan spared Anakin despite knowing killing him would "burn his heart to ash" and feeling regret for what Anakin could have become. Obi-Wan's decision to spare Anakin was influenced by the dark feeling that accompanied Emperor Palpatine's arrival, preventing him from finishing him off.
A year later, in 4 ABY, with his health rapidly declining, Yoda died of old age when Skywalker returned to see him. He became one with the Force, shortly before Luke redeemed his father Anakin, who fulfilled his destiny as the Chosen One by killing Sidious.
According to McCallum, he believes that Anakin being as good as dead when Obi-Wan leaves Mustafar is the reason that he did not feel there is any reason to actually deal the final blow to the former Jedi.
Instead, the theory suggests that Padmé died because Palpatine killed her. By taking a closer look at how Anakin's transformation into Darth Vader is intercut with Padmé's death, a clearer picture emerges of how Padmé's death was more likely a planned-out murder as opposed to an accidental tragedy.
Could Obi-Wan have beaten Vader in A New Hope on the Death Star if he really wanted to? Most likely, yes. Obi Wan had already demonstrated that he was a comparable opponent for skill with a light saber on Mustafar where Obi Wan beat Anakin.
And all the way up until the third sister told him otherwise, Obi-Wan truly and completely believed that Anakin was dead. He believed that he died on the banks of Mustafar. And even if they knew for a fact the Anakin had survived, it comes back to the split personality.
On a practical level, Obi-Wan did not know that Anakin had survived simply because he was isolated on Tatooine. Although Obi-Wan did not see Anakin die after their battle on Mustafar in Star Wars: Revenge of the Sith, he reasonably could have assumed that Anakin would die from his injuries.
Star Wars: Tales of the Empire added three new Jedi to the list of those who survived Order 66: Barriss Offee, Lyn Rakish (the Fourth Sister) and an unidentified human Jedi. The unidentified human had never been seen before in Star Wars, and they made their debut in Tales of the Empire.
In his eyes, removing any kind of hindrance (including killing younglings) that would block his path from saving the woman he so deeply loved was the only way. Sadly, his love for her sealed his fate.
He meant that he "killed" that part of him. Anakin as in the jedi, the compassionate, good person he was is nowhere to be found after he turned to the dark side.
Did Darth Vader kill Anakin Skywalker? It can be argued, from a certain point of view, that Vader made the same assumption as Obi-Wan did. When Obi-Wan left his former friend and apprentice to die on Mustafar, he assumed that was the end of it. That both Anakin Skywalker and Darth Vader had perished.
This fell in line with various fan theories and non-canon novelizations that suggested Obi-Wan and Padmé had an affair sometime during the Clone Wars. In the Star Wars prequel movies, Obi-Wan often has a duty to protect Padmé, but there was never any sense of romantic feelings.
He had killed his master, Qui-Gon Jinn, for which Obi-Wan got his revenge by slicing the Sith in half. But after Maul took yet another person away from him, Obi-Wan absolutely hated Maul. The Jedi Master almost let his rage and anger get the best of him, growing close to the dark side before he rejected it.
However, in Obi-Wan Kenobi episode 3, Kenobi spends most of his time trying to evade Vader despite this earlier victory, signaling his intense fear at once again meeting his former padawan.
