Marvel's Magneto is one of the most compelling villains ever produced by the company; as the ever-present opponent of Professor X and the X-Men, Magneto is a champion of mutant rights around the world. His backstory as a Holocaust survivor shaped him into the man he eventually became, and molded his worldview. However, there was a period of time in which Magneto worked alongside none other than the Red Skull, enemy of Captain America and a card-carrying Nazi - and Marvel did their best to make sure readers forget about the entire affair.
Initially, Magneto was simply a villain who believed that mutants are better than humans, and thus fought against the X-Men for standing in his way. Though formidable, Magneto would not receive his backstory until Chris Claremont took over the title in the 70s. In Uncanny X-Men #150, Magneto was confirmed to be Jewish - and a survivor of Auschwitz, one of the most horrible of Hitler's concentration camps. With the atrocities of the Holocaust fresh in his mind, one would think that Magneto would rather work with anyone else before he'd sit at a table with a Nazi like the Red Skull - and yet, that's exactly what happened in the Marvel crossover event Acts of Vengeance.
In Acts of Vengeance, Loki (in the guise of an ordinary human, though he quickly drops the act) gathers multiple powerful villains from different corners of the Marvel Universe with the intention to destroy the Avengers once and for all. The Kingpin, Doctor Doom, Magneto, the Wingless Wizard and the Red Skull all answer Loki's call, who plans to engineer a mass breakout of minor villains to overwhelm the Avengers and eventually lead to their downfall. The villains bicker with each other - as villains are wont to do - yet despite everything the Red Skull has done (including being taught by Hitler himself), Magneto still works with him...at first.
Fan outrage over the fact that Magneto would even stand to be in the same room as the Red Skull, let alone team up with him, eventually led to the events in Captain America #367, written by Mark Gruenwald (who, despite passing at the young age of 43, wrote more issues of Captain America than any other writer in history). Magneto captures the Red Skull and throws him in a bunker with only a small amount of water and no food. Magneto seals the bunker, fully intending for the Red Skull to die of starvation. "I want you to suffer as you've made others suffer. I want you to wish I had killed you."
Unfortunately, the Red Skull would be rescued by Crossbones and live to fight again, but the one moment in Captain America #367 seemingly redeems Magneto's team-up moments in Acts of Vengeance. Magneto has witnessed atrocities at the hands of the Nazis; for him, to work with a Nazi is to shake hands with the Devil. Magneto is a captivating villain because he does not see himself as one, but rather as a freedom fighter, crusader, and champion of mutantkind - and those who stand for hatred and bigotry like the Red Skull are not even worthy of a quick death.