One of the most tyrannical sith to have ever burdened the galaxy, Darth Vader's numerous crimes against the Star Wars universe make many reticent to forgive him, even after his betrayal of Emperor Palpatine and redemption in the eyes of his son.
Just as there are reasons to condemn the Dark Lord, there are also causes for his forgiveness. By weighing the best parts of his absolution against the worst, we can accurately assess whether or not redeeming him was a wise narrative decision through the context of his descent into the dark side and subsequent return to the light.
10 Stayed Evil - Killed Younglings
If there's one threshold too egregious to forgive, it is the murder of young children. Shortly after Anakin's betrayal of Mace Windu, he would march to the Jedi Temple, where even the most harmless, innocent souls would not be spared of his wrath.
Given that he was willing to slaughter these kids without remorse so recently into Sidious's service, it stands to reason that he might have always had some capacity for this level of evil. Therefore, it concludes he should have remained on the dark side.
9 Redeemed - Ahsoka
Though Ahsoka may have been Anakin's padawan, she was a fantastic source of stabilization for him. Poetically, she would often make decisions closer to that of a traditional Jedi than he did, including sparing the life of Darth Maul right before Order 66 was enacted.
She would not give up on her master after his fall, confronting him and buying other rebels time to escape. Given the courage and perseverance she showed to her former teacher, it would be fulfilling to her character for him to be redeemed, rather than slain as an enemy.
8 Stayed Evil - Complacency
Darth Vader was complicit in many of the Empire's atrocities, including the destruction of entire planets and the enslavement of workers. The only time he would lash out against it is when his own son would be endangered ~ which can be interpreted as a selfish reason.
Given that he never objected (often even leading) the worst crimes the galaxy had ever seen, he tacitly approves of them and the countless lives they have ruined. Vader is more than simply a commander to his stormtroopers ~ he is their symbol of strength and the pillar they rally behind to feel secure in their actions.
7 Redeemed - The Jedi Way
One of the essential themes of the Jedi was that of forgiveness, a practice they have employed infrequently. Since the story of Darth Vader began in the franchise's infancy, it was imperative that this was exemplified as a core principle of the order.
Luke's decision to spare Vader would juxtapose Anakin's own choice in beheading Dooku. Ironically, the Emperor would be present in both situations and demand the young Skywalker's compliance. By refusing to and redeeming his father, he broke the cycle of hatred and boldly defied the strongest sith in the galaxy.
6 Stayed Evil - Already Had A Chance
When Obi-Wan heard about what Anakin did and confronted him for it, he was not immediately prepared to take off his head. Alongside Padme, he attempted to reason with him, even begging him not to continue the fight before he chopped off his limbs and left him for dead amidst a lake of fire.
If Anakin were to be redeemed, this would have been the time for it. He might not have ever returned to the Jedi Order, but given his proximity to Sidious, he would have been able to end the madness before it spread. He ignored Kenobi's offer and attacked Padme instead.
5 Redeemed - Ghost Approval
When determining whether or not one should be forgiven, it is often prudent to consult the offended party. During the celebration on Endor once the Empire had been defeated, Anakin's ghost appeared alongside Kenobi's and Yoda's.
Since they were evidently accepting of his company, this entails that they had forgiven him for the grievous wrongs he had dealt them in life. While others may not have felt similarly, Yoda and Obi-Wan lived to feel the consequences of Vader's actions the longest.
4 Stayed Evil - Massacring Rebels
The Jedi's forgiveness is not the only one Vader would need. He was directly responsible for butchering countless rebels, with an excellent window into his depravity seen in the concluding action of Rogue One.
With how many widows he's personally created and the heartache he has caused the rebels through his direct malevolence, it's difficult to imagine him ever being fully exonerated from what he has done, let alone portrayed as a decent person with a heroic end.
3 Redeemed - Brought Balance To The Force
Anakin Skywalker's destiny has been to bring balance to the Force, and technically, he did so through both his betrayal and redemption. At the conclusion of the Clone Wars, the Jedi were inordinately powerful and had almost completely overrun the combined Separatist and Sith resistance. By the end of the original trilogy, Luke was the only of his order left.
By "killing" Palpatine, Vader was fulfilling what had been written for his destiny, and subsequently, his actions might be forgiven since they were predetermined.
2 Stayed Evil - Became Sith For A Selfish Reason
The tipping point at which Anakin became Darth Vader was when Sheev Palpatine promised "he had the power to save the ones he loved." Superficially, this might not seem selfish, as he is caring for someone other than himself.
Nonetheless, he was still willing to betray his principles for power, as demonstrated through his fanatical devotion to Sidious's bidding. He would not have a respect for life, instead solely fixated on Padme's safety and health. Ironically, his obsession with her would result in her death as a consequence of his callousness.
1 Redeemed - His Mother
Shmi Skywalker was one of the few people that cared for Anakin before the Jedi's arrival. She was his mother, friend, and guardian, providing hope where Watto did not.
Anakin left Tattooine under the premise that she would one day join him, but since the Jedi were too preoccupied to deal with her, she was taken and slaughtered by Tuskan raiders. This would cultivate a sense of righteous indignation in the young man, one which Obi-Wan would not correctly staunch through his reverence of the flawed Council's judgment. It provides us a sympathetic reason as to why he turned against his allies.