The villains in the MCU are a surprisingly great reflection of their comic book counterparts — although cinema has a habit of pushing at the boundaries of character traits, motivations, relationships, and so on, they have maintained an incredible similarity with the origin stories. But this isn't necessarily the case all the time.
Every fan may not enjoy the specific depiction of their favorite hero or villain, for instance, for altering their narratives in a minor but significant manner. Some of them might appear differently, or they have unrelated careers, or maybe they aren't supposed to be in a particular era or hanging out with certain other characters.
Like Their Comic Counterpart: Thanos
Josh Brolin's Thanos does not need any kind of adjustment, attitude, or otherwise. He is arrogant, manipulative, brutal, and has no qualms regarding the death of countless living beings, as long as it suits his purposes.
In both versions, he brings about a cosmic genocide, although it must be noted that his reasoning is different. In the comics, Thanos desires nothing more than getting Lady Death's attention, while in the films, he states some vague ecological rationale for his actions.
Could've Been Better: Ronan The Acc
In the context of appearance, Ronan the Acc (except for a skin-tone modification) appears pretty much the same, including his iconic scalp cloak and his weapon.
The difference is that his Universal Weapon, the Cosmi-Rod, is strong enough to make explosions of energy, create barriers, help in flying, manipulate elements, and a number of other abilities, without the need for the Power Stone. The films could have borrowed more from the comics in this case, as giving him a backstory as a Kree Acc requires explaining the history of the Universal Weapon a bit more.
Like Their Comic Counterpart: The Winter Soldier
The Winter Soldier, or Bucky Barnes, is very similar in either form of media: being a cold murderer because of the brain-reprogramming experiments he is subjected to.
Another parallel is the relationship between him and one of his closest compatriots, Steve Rogers. Sebastian Stan brings his character to life quite well, a feature especially noticeable in the complexity of Bucky's suffering throughout his arc.
Could've Been Better: Hela
Cate Blanchett got the Ancient One spot on, but her performance of Hela (while sensational), doesn't have the comic background. The biggest change, one which could have worked in spectacular fashion, is her origin story: the daughter, not sister, of Loki.
In fact, Hela is mentioned as such in Norse mythology, as well. Maintaining this relationship is core to her character, because it usually involves her and her father working together towards similar goals.
Like Their Comic Counterpart: Loki
There is scant chance that anyone else will be able to become Loki as well as Tom Hiddleston, even if it deviates from the multiple variants of the character in the comic narrative.
His trademark charm combined with a flair for deception is all he needs to be the thorn in his brother, Thor's side. The one thing that is different is that the comic Loki is forced into his villainy, so to speak, because of Odin's clear bias towards his elder son, while the movies show him as being "evil" because of his fraternal envy.
Could Have Been Better: Ego
Ego has gone through a momentous transformation from comics to film, in his presentation, for one. The first version is actually a planet with a humanoid face, rather than one which only manifests itself as Kurt Russell. Further, Peter Quill's father is a completely different entity (J'son of Spartax.)
The reason for this modification is understandable, as it provides a new villain for audiences without having to deal with the increased production budget required to reflect Ego's true form in CGI, but it takes away from the pathos of the character.
Like Their Comic Counterpart: Erik Killmonger
Erik Killmonger is burdened by vengeance, on of his father's banishment from his homeland of Wakanda. His backstory is nearly identical in both cases, from his anger directed towards T'Challa for his situation to his calculated planning to seize control of the nation.
Only, the film character is from California, while the comics begin in Harlem, NYC. Michael B. Jordan adds a considerable amount of pizzazz to his role, but that's been an asset, to be honest.
Could've Been Better: Nebula
Nebula is supposedly Thanos' "adopted daughter", which is what binds her to the supervillain in the MCU. On the contrary, she has no genetic link to him in any way, although he has referred to her as his granddaughter (a remark that remains unconfirmed).
More importantly, though, is the fact that Nebula plays a central role in the Infinity Gauntlet comic series, as she manages to get her hands on the item and takes total control of the universe with the help of the six stones, requiring Thanos to team up with Dr. Strange, Adam Warlock, and the Silver Surfer.
Like Their Comic Counterpart: Red Skull
The comic book version of Red Skull, a major Captain America nemesis, is a very accurate rendition in cinematic form. His appearances are practically indistinguishable, largely relating to his past as one of the most powerful Nazi officers serving under Hitler.
The one small difference is that the original character's looks are based on him accidentally "drugging" himself with the Dust of Death, his favorite chemical weapon, while the film one's face morphs into a red skull upon taking the Super Soldier serum.
Could've Been Better: Mysterio
Mysterio has always been skilled in creating illusions, with the help of chemicals and mechanical engineering, mostly the former. The movie version, on the contrary, uses a completely different take on the character; Quentin Beck employs digital means to further his cause, making holographic illusions with the help of a horde of drones.
Another major divergence occurs in Mysterio's personality: Jake Gyllenhaal's personality is considerably more warmhearted and commiserative than in the comics, which takes away from the villain's classic conniving nature.
