I think that if polaris breaks magneto out, she will do it because she is against the government. First, the government doesn't have the best track record regarding mutants. Second, her boyfriend just dumped her to be a puppet of the government. I think that she will see magneto as a political prisoner not a murder and feel that breaking him out is the right thing to do.
Although, my biggest wonder is what magneto is going to be like after he escapes. Part of me thinks that he will see himself as a mutant Nelson Mandela and want to be a political activist. It just wouldn't seem right to have him go on a killing spree upon freedom. I am also a little worried that they are going to have magneto escape and be recaptured in the same arc in which case he won't have the opportunity to do anything. Which I think would be stupid.