Bucky is a particularly interesting character to analyze in light of the decisions made in Captain America:The Winter Soldier that changed him from the comics winter soldier.
These changes from comics canon contain some of the things about the character that were compelling, and also the things MCU had no idea what to do with in later installments
In the winter soldier comics, (which are themselves a violent re-invention of the character, he was raised on a military base and became Steve's sidekick after Steve had become Captain America, kind of a darker figure willing to do dirty work that Cap couldn't be seen doing
in the movie, he's Steve's closest childhood friend. They only end up paired up and fighting together because Steve goes on a desperate mission to save his life
in the winter soldier comics, he is something like 7 or 8 years younger than Steve and they still have a mentor/sidekick type of relationship
in the movie they are the same age and steve is no longer a "mentor" figure, that dynamic is eliminated
in the winter soldier comics Bucky loses all his prior memories after his apparent death, making him a blank slate to be groomed into a soviet super-assassin. There is no brainwashing.
in the movie they deliberately erase his memories by strapping him into this scary device that fries his brain with electricity. It's clearly torture: he is shown hyperventilating as the restraints close onto his limbs and then screaming in agony as the device activates.
in the winter soldier comics Bucky as the Winter Soldier is capable of independent thought and snark, and is shown questioning and mouthing off at his superiors
in the movie, Bucky is completely passive. He barely speaks at all; when he does, he is almost childlike, meek and quiet in his interactions with the Hydra characters, stubborn and confused in his fight with Steve. The main antagonist slaps him across the face for not answering a question and he doesn't retaliate at all even though he can obviously kill everyone in the room in the blink of an eye. In the same scene he also lets the scientists manhandle him and eagerly opens his mouth for the mouthguard even as his heart rate is spiking on the monitor and he's starting to hyperventilate because he KNOWS the pain is coming.
(side note: he is shirtless in this scene for no reason)
(second side note: the line "who the hell is Bucky?" is in the movie because it's iconic from the comics, but it's arguably super OOC for mcu!bucky)
The long hair and cyborg arm are straight from the comics, but the most striking change to his appearance is his mask: in the comics, he's wearing a domino mask over his eyes, but in the film, he has an opaque black mask covering his nose and mouth that takes away much of his ability to emote and looks strikingly like a muzzle. The comics mask evokes mysterious wiles; the film's mask evokes dehumanization.
basically the films gave him a much deeper and more intimate connection to Steve while putting the two of them on even footing as friends and partners, and changed him from a morally gray character who indifferently kills people and regrets and becomes angsty once his memories are restored, to a tortured and dehumanized human weapon who obeys despite not understanding anything that's going on because he knows nothing but pain and punishment.
The film's version is really much more interesting. Snarky antiheroes who kill indifferently are a dime a dozen; a character who is palpably, terrifyingly dominating and powerful yet completely powerless in the hands of those who control him, who is hollowed out of all personal identity and who has no agency or control over his own body as it is mutilated, reconstructed and wielded as a weapon, is something much more delicious and fascinating.
We watch this guy slaughter people effortlessly with an apex predator swagger that projects pure dominance and prowess, then we watch him meekly accept abuse and torture with soft, confused eyes.
Of course I'm insane about him. There's a lot to be insane about.