Magneto could easily be considered the X-Men's archenemy and is one of Marvel's most enduring villains.
What puts the Master of Magnetism head and shoulders above most other super-criminals is how complex his character is. As a Holocaust survivor, Erik Lehnsherr has seen the worst of what can happen when one race considers themselves superior to another. Because of this, Magneto fights to make sure that mutants are never subjugated by mankind using whatever means necessary.
This noble motivation has seen Magneto earn a measure of respect from the X-Men and he has even joined their side on numerous occasions. However, Magneto is still a supervillain who isn't afraid to get blood on his hands if it gets the job done. His extremist views on how to best protect mutantkind have led to him committing some pretty heinous acts over the years.
While some of these could be justified as necessary evils for the greater good, others demonstrate an inherent hypocrisy in Magneto's actions. He is often willing to inflict the same abuses on humans that he fights so hard to protect mutants from.
And then there are the times he just straight up tries to wipe out all life on Earth.