Most Star Wars fans agree that the definitive moment Anakin Skywalker 'became Darth Vader' occurred when he murdered Younglings--future Jedi, but still innocent children--in cold blood. But the Darth Vader comic series actually reveals an incredible scene in which this pivotal transformation transpired after Revenge of the Sith.
Before The Empire Strikes Back, Vader makes the unpleasant discovery that Emperor Palpatine enlisted the creator of Vader's robotic armor, the technician Cylo, to participate in a blasphemous endeavor: building lightsaber-wielding cyborgs without any Force sensitivity to replace him. When Darth Vader proves himself to Palpatine as he so often does in the larger Star Wars canon, Cylo attacks the Empire in retaliation for being so carelessly disregarded. But by attacking Vader, Cylo actually helps the Sith Lord to truly be born.
Cylo's foolish act of aggression against the Sith Lords appears in 2015's Darth Vader #24 by writer Kieron Gillen, artist Salvador Larroca, colorists Edgar Delgado, and letterer Joe Caramagna. While fueled by rage, the long-awaited showdown appears to end unceremoniously as Cylo trigger's Vader's kill switch, deactivating the life-giving cybernetic portion of Vader's armor, bringing the Sith Lord to his knees. As he kneels seemingly lifeless, the reader is brought into Vader's mind as he reimagines pivotal moments of his life in a thrilling spiritual journey of death.
First visiting Obi-Wan cutting off his legs on Mustafar, only this time, instead of just expelling random hateful words at his master in fits of rage, he convinces Obi-Wan to throw him into the lava. From the fiery inferno emerges the robotic form of Vader, who immediately proceeds to kill Obi-Wan before facing the form of Anakin Skywalker himself. Vader quickly slays him in a lightsaber battle, before facing the greatest obstacle of all: Padmé Amidala.
The altered reenactment of her initial confrontation with Anakin before Vader's duel with Obi-Wan sees the Jedi Force choke yet again... only this time, Obi-Wan isn't there to save her. And Vader finishes the job, causing him to awaken in the real world and cut down Cylo.
It's a stretch to say Vader fully 'became a machine' in that exact moment, eliminating the remaining organic parts of him physically, rendering his dependency on the suit's cybernetics irrelevant--meaning the transformation is more effective on a figurative level. As Obi-Wan Kenobi famously says in A New Hope, "He's more machine now than man, twisted and evil." Kenobi could have been referencing the mechanized parts of Vader, but his use of "machine" also speaks to the state of Anakin's emotions, and soul. As Anakin, he loved Padmé desperately and clearly struggled with the realization that he killed her after awakening as the mechanized version of himself for the first time. The fact that Vader could now, either figuratively or literally, revisit a moment when Padmé lived and still kill her is an act of shedding his remaining humanity.
This particular storyline causes complications for Vader's later in-canon series, as future writers must pretend this momentous transformation never occurred, eventually sending Vader to uncover the secrets of Padmé death (a desperate search for truth which, if this story still informs his character, doesn't quite add up). The scene is an unforgettable one for fans of Darth Vader, but given the stories and his actions still to come, it's a bit too inhuman, even for him.