I found McKellan to be a good piece of casting as Magneto because he exuded the humanity and gentleness of Magneto.
Magneto's a "believer", he is the definition of "ends justify the means" pragmatism.
such people are GOOD people, they're caring, loving and passionate, and they're DANGEROUS because of it.
evil is not some robotic/analytical type, it's good intentions, it's egocentricism, it's arrogance and overreaching.
that's what Magneto is. he see's the obvious stupidity in Xavier's method and creed, but instead of trying to find a more reasoned and rational position, he takes a high horse morality of protecting "his people" against the stupid and insecure homo-sapien.
nobody would follow Magneto if he didn't have a kernal of moral truth and "rightness" to his position, it's just that he becomes too passionate, too righteous and too blind to the consequences.
like the witch hunters, the inquisition and fundamentalists... perverted goodness is far worse and far more potent than outright wickedness.