Across all of their projects, including the shows, the Marvel Cinematic Universe has introduced over a hundred villains for the heroes to battle against and although they have gotten criticism for how their antagonists are handled in the past, some have stuck around and become fan favorites.
However, while some of those villains have managed to be reform themselves, becoming allies of the heroes or even choosing to become heroes themselves, others are too far gone to be redeemed and most wouldn't want to be.
Irredeemable: Kaecilius
The best villains see themselves as the heroes of their own story rather than the antagonists of somebody else's, which is why it makes so much sense that Kaecilius was so intent on bringing the dark dimension to Earth.
In his mind, he wasn't doing anything wrong but was rather offering people a chance at living forever, free of death, causing him to see his actions as noble. Even if he wanted to seek redemption for almost destroying the planet, it's too late for him since he and the other zealots were sucked up into the dark dimension by Dormammu to spend eternity with him.
Redeemed: Loki
Since Loki is the God of Mischief, The Avengers while leading Thanos' army in an attack on New York.
However, after the death of Frigga in Thor: Ragnarok after the two shared a heart-to-heart over the death of Odin.
The final moment of Loki's redemption arc came in Avengers: Infinity War when he was killed by Thanos for betraying him while protecting Thor. While he wasn't perfect, the fact that he gave up the Tesseract rather than see his brother die shows just how far he came.
Redeemed: Nebula
In Guardians of the Galaxy Vol. 2, after attempting to kill her sister and finally managing to beat her in a battle, Nebula forgives her and helps the Guardians in their fight against Ego, before setting out to get her revenge on Thanos.
She worked with the Avengers to fight Thanos in Avengers: Infinity War after learning that Gamora was killed on Vormir in an attempt to spare her life. Then, in Avengers: Endgame, she and Rocket ed the Avengers during the five-year gap since they were the only surviving of the Guardians. It seems that her path to redemption is complete since she reed the team not long after.
Irredeemable: Hela
In Thor: Ragnarok, fans got introduced to Hela, Thor's long-lost sister that he didn't even know about. Hela believed that she was the rightful heir to the throne of Asgard and was willing to kill anyone that stood in her way, including the Warriors Three.
Considering she is the Goddess of Death, there is a chance she could return despite seemingly being killed by Surtur at the end of the movie. However, the fact that she lost the throne not once, but twice would probably just make her even madder, especially since she had a level of entitlement that hundreds of years locked away couldn't change.
Redeemed: Scarlet Witch
Wanda Miximoff was introduced in Avengers: Age of Ultron after she allowed HYDRA to run experiments on her, using the mind stone which unlocked her abilities. After the Avengers shut HYDRA down, she then aligned herself with Ultron due to her hatred for Tony Stark (since his weapons were responsible for the death of her parents).
She used her mental manipulation powers to give the Avengers visions and send Hulk rampaging through Africa before he was stopped by Iron Man. However, she decided to switch sides after learning about Ultron's plan for the destruction of the world.
After a speech from Hawkeye, she became an official Avenger and was the one to deliver the final blow to Ultron by ripping his heart out. Captain America: Civil War, Avengers: Infinity War, and Avengers: Endgame.
Redeemed: Quicksilver
Like his sister, Pietro was also introduced in Avengers: Age of Ultron and allowed himself to be subjected to HYDRA's experiments, initially fighting alongside Ultron against the Avengers. However, he soon ed the Avengers upon learning the robot's true intentions.
During the final battle, he used his speed to fight Ultron's drones and help civilians get to safety. He then sacrificed himself to save Hawkeye and a young boy he was trying to rescue by using his body to block the bullets being fired at them by Ultron. It seemed that Pietro had redeemed himself in the eyes of Hawkeye since he went on to name his youngest son after him.
Irredeemable: Red Skull
While most MCU villains became bad guys because of some sort of personal tragedy that led them to the dark side, Johann Schmidt is the MCU's embodiment of pure evil. As the leader of HYDRA, Schmidt was seen to directly work with the Nazi party and participated in several unethical experiments. He was also seen to be responsible for the deaths of thousands of people.
Out of all the villains in the MCU, it's not hard to see why the Red Skull is unredeemable as no amount of time could fix the horrors he caused on Earth. Even the tesseract viewed him as unworthy and banished him to Vormir.
Redeemed: M'Baku
While he wasn't a complete villain, M'Baku starts off an ally to the royal family when they recover T'Challa's body from the waterfall after his fight with Killmonger, preserving his life by keeping him on ice.
He and the of the Jabari also ed the final fight against Killmonger and the Border tribe, despite M'Baku originally saying he wouldn't get involved. He and the Jabari also stood alongside T'Challa against Thanos' outrider army in Avengers: Endgame.
Redeemed: Winter Soldier
Bucky Barnes is one of the few MCU characters to have a fall and redemptive arc. He debuted in Captain America: The Winter Soldier, where it was revealed that he survived before being recovered by HYDRA and brainwashed to be an assassin.
After the fall of SHIELD, he moved to Romania and his memories of the past started coming back to him. At the end of Civil War, he was taken to Wakanda to be healed by Shuri and was fully cured of his programming by the end of Black Panther.
Since then, Bucky has tried to redeem himself in the eyes of the Avengers, helping them in the Battle of Wakanda, the Battle of Earth, and later helped Sam Wilson take down the Flag Smashers.
Irredeemable: Ego
When Peter first meets Ego in Guardians of the Galaxy Vol. 2 and finds out that he's actually his long-lost father, it seemed like a dream come true for the character. However, that dream quickly became a nightmare when Peter discovered that Ego had killed thousands of his offspring after learning they didn't have Celestial DNA before also revealing that he knowingly put the tumor in Meredith Quill's head that would lead to her death.
While it would have been interesting for Peter to get to keep his god-like Celestial powers and have his father be a part of his life, there was no way Peter was going to let him live after what he did.
