Vader said it himself. He wanted Kenobi to suffer. That's why he dragged him into the flames instead of just killing him — so that Kenobi could feel a bit of what he himself felt as a limbless crispy critter on Mustafar.
So instead of risking letting his emotions get the better of him, Darth Vader opts to let Obi-Wan go with the assumption that he is no threat to him or the Empire.
But this epic showdown ended on an incomplete note, with Vader allowing Kenobi to escape with his life as a wall of fire separated the two characters.
Visions of Obi-Wan. The comic opens during the last moments of Revenge of the Sith, recreating the well-memed instance when a despondent, freshly-armored Vader cries out with a drawling “nooooooo” over how much he's lost.
Vader believed that his and Padmé's child died when she did, so there was no reason for him to consider anything else.
His suffering meant he always expected the worst in people, but because of this, it caused his biggest flaw. This mentality stretched back to when Anakin was a Padawan, as seen in Part V of Obi-Wan Kenobi. As Anakin and his master train at the Jedi Temple, he's able to overwhelm Kenobi and disarm him.
Darth Vader was the stronger of the two by far, but in most cases with lightsaber duels, the victor is the one with the strongest connection to the Force. Obi-Wan finished his last fight with Vader by hurling a barrage of rocks, and it's unlikely he had lost this level of power by A New Hope.
In Star Wars Legends and Canon, Galen Marek (Starkiller) is the only Jedi (other than Obi-Wan Kenobi) to put a beat down on Darth Vader/Anakin Skywalker. Galen was secretly trained as a Sith Lord apprentice to Vader, but returned to the light side and the Jedi.
The duel. Darth Vader defeats Darth Maul. The two Sith Lords engaged in a lightsaber duel, going through the volcano and onto a bridge situated over a river of lava, which collapsed, though they both survived and continued their battle.
As soon as Anakin Skywalker became Darth Vader, he already faced his most persistent nemesis and lost, namely his own master, Obi-Wan Kenobi.
This information comes to light form the comic series Darth Vader: Dark Lord of the Sith by Charles Soule and Giuseppe Camuncoli, in particular issue #7. Palpatine admits to Darth Vader that the Jedi he actually feared most was none other than Jocasta Nu.
Admiral Ozzel : [appearing onscreen with Captain Piett] Lord Vader, the fleet has moved out of lightspeed and we're preparing to... Darth Vader : You have failed me for the last time, Admiral! Captain Piett?
In that time Anakin apologizes to Obi-Wan, desperately asking him to forgive him for everything he has done to him and everybody over the years. Obi-Wan's response was teaching Anakin how to do connect with the force and keep his spirit intact.
In the canon Star Wars book, The Princess, the Scoundrel and the Farmboy by Alexandra Bracken, Darth Vader senses the Force within Princess Leia when he is interrogating her during A New Hope. Princess Leia resists Vader's probe, revealing her Force sensitivity to him.
Because he was not an active member of the Jedi Order during Order 66, Infil'a escaped execution. Some time later, he faced the Sith Lord Darth Vader wielding a green lightsaber. Infil'a put up a fierce fight against Vader, almost killing him by throwing the Sith Lord off of a cliff.
His armor could be hacked and controlled by a device. In a turn of events, Darth Vader's right limb holding his dangerous scarlet lightsaber was moving of its own accord toward his neck. This was a dangerous weakness indeed.
During the Imperial Era, a human female nurse served the Galactic Empire on board the Death Star under the supervision of a doctor. They both served the Sith Lord Darth Vader as his personal medical caretakers. The nurse became obsessed and psychotically in love with the Dark Lord of the Sith.
Darth Vader's Suit Embeds Needles In His Skin
Inside his suit and mask, Vader has a series of needles in his skin. Unfortunately, these needles, which transmit neurological data throughout his body and allow him to control his limbs, also cause him constant pain.
Between Darth Vader and Obi-Wan, it's pretty clear that Vader is far more powerful with the force. After all, he pretty easily used the force to stop a starship from taking off in the previous episode, and manhandled Kenobi throughout the majority of the show.
As seen time and time again, the Sith Lord could easily cut down almost anyone who got in his way, and he rarely ever lost a fight. Yet at the end of Star Wars: Return of the Jedi, Luke is able to beat Vader to the ground with relative ease, and so what was different from all of Vader's previous fights?
Physically Vader is stronger and tougher while Dooku is faster. In terms of duelling Dooku's ability to take on Yoda for a while and being a peer with Mace is superior to anything Vader has achieved. In terms of force powers Vader's scaling above people like Galen Marek and Starkiller puts him ahead of Dooku.