I think Magneto fails to qualify because, despite being an antagonist, he’s not really a villain in the sense that the audience fears for the well-being of the heroes when they’re around him.
When Vader shows up, your heroes are in trouble! Joker walks into a room- the unpredictability creates tension. He kills and schemes just to toy with Batman and test his will. Homelander is riveting- every time he’s on screen you wonder what the hell is this maniac gonna do. Thanos in Infinity War had a similar presence. He was so in control of every situation that once he’s in the vicinity, you fee the sense of helplessness of all other characters who have to fight with all they have just to have a hope of even slowing him down.
Magneto is more likely to have a philosophical discussion with Charles than hurt him so we rarely fee the sense of threat when he’s around.
He’s just one step beyond an antihero in the movies. He’s been terribly wronged in many ways and his vengeance and ideology are completely justified; the heroes mostly just disapprove of his approach.
The people Magneto goes after totally deserve it. Who doesn’t want to watch him slaughter the Nazis who abused him as a child or the political class who created policies and machines of death to eradicate his kind, the scientific, military facility that’s producing a so called ‘cure’ to his existence? Kill em all!
X1 is the only time he was the real villain of the story but, even then, his scheme was not particularly evil. He wanted to convert world leaders into mutants so they would become more sympathetic to his kind. A little extreme but still rational given that mutants are so violently discriminated against and have no political support.
He’s an excellent character and a great ideological antagonist to the X-Men and Charles’ philosophy of peace, cooperation and coexistence through acceptance. He’s just not been presented as a villain to be feared.
