It's not like she goes around all the time trying to wipe out humanity.
So the implication there is that she should get her first shot at genocide free, and it should only count she came back for a SECOND time? Seriously?
And you surely can't be suggesting she is selfLESS? Mystique IS selfish, and one sure sign of it is how she's so damned manipulative. Even her relationship with Rogue has always been dominated by HER needs, HER belief in what is right, and completely over-rules what Rogue might have wanted. Putting your own desires ahead of what your child needs, that's supremely selfish.
She may dress her actions up in "I'm acting against the humans who oppressed my genetic kin", but when it comes down to a choice between what is good for them and what is good for Mystique, I'm having a hard time she's ever made the truly hard choice.