Magneto has always been a complex character. Throw in the fact that he's a major character in X-Men: First Class and that complexity gets amped up even further. Again, the point here isn't to complain that the movie is in its own continuity (just like Ultimate Comics are different than the 616-Universe). We want to look at who is Magneto in the comics, what do the (many) trailers tell us and how would we like to see him portrayed in the movie.
Magneto was supposed to have been born in 1929. With the movie taking place in 1963, that works. He was born to a German Jewish family and they were killed after they fled to Poland. He escaped but was captured and sent to Auschwitz. Here, he was reunited with a girl, Magda, he had fallen in love with while he was young. They escaped and this is when he changed his name to Magnus. They had a daughter that was killed, which sparked his anger against normal humans.
Changing his name to Erik Lehnsherr, he eventually met Charles Xavier. They discussed their views on the evolution of man and mutantkind but soon realized their ideals differed too much. Erik became Magneto and has been both an enemy and ally to the Xavier's team of mutants, the X-Men.
How much of this will carry over into the movie?== TEASER ==
The main thing is we're not clear on whether or not this movie is actually in the same continuity of the other X-Men films. The trailers have used a couple images in the "Before he was Magneto" portion. It's clear that he will still have his same childhood. This has always been a major factor in establishing who he is and who he will become. It's probably safe to assume that he did not have a child that died or became the father of the Scarlet Witch and Quicksilver before meeting Charles Xavier.
The biggest difference is Magneto remains by Charles' side longer than he did in the comics. Magneto is there at the beginning of the X-Men's formation. In the comics, it wasn't until Uncanny X-Men #200 that he put aside his evil ways and agreed to become headmaster and teacher for Xavier's younger team, the New Mutants. This movie shows us a point in Magneto's life where he could still take his destiny in the direction of good or evil.
Assuming he's willing to play by Xavier's rules in order to be part of his team, something must happen that forces Magneto over the edge. Taking place in 1963, we've seen the team takes the blackbird and goes into enemy territory. With the power and abilities Magneto possesses, he can easily dispatch a threat. In one scene, a guard comes at him with a knife and Magneto forces the guard to stab himself. Xavier may not be completely prim and proper but I don't think he would approve of killing unless there was no other option.
There's also the question of Mystique's role. If this movie does follow the timeline of the other movies, we know she remains one of his loyal followers. Mystique's journey in this movie seems conflicted. She appears to be an innocent young mutant trying to figure out her place in the world and on Xavier's team. We have seen in a TV spot that she meets a young Charles Xavier as a child, which puts them at about the same age. It's unclear whether or not Xavier is aware the blonde and human looking Mystique is the same blue-skinned girl he met as a kid. But Magneto discovers her secret. Does he keep it from Charles, thus strengthening her loyalty to him or is Charles already aware?
X-Men: First Class might be about Xavier and the formation of the X-Men but it's clear that a big part of this movie is dedicated to Magneto. We will see him transform from friend and (possibly) co-creator of the X-Men to the red helmet-wearing mutant terrorist. Seeing what could be the one incident that pushes him over the edge is something I'm looking forward to seeing in this movie.
It feels like this needs to be a longer movie. To have him go from Charles' friend to wearing his gray helmet to finally wearing the entire red ensemble seems like something that would take more than one movie. Then again, the movie is reported at being 140 minutes. Speculation is they are betrayed by the U.S. government but that doesn't seem to be enough of a reason. What is it that makes him turn on Xavier and the ideals he agreed to? Who betrays him and how much does Magneto lose or suffer to become a terrorist?