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I suppose it depends on how much you believe people can change. While I definitely believe people can see the error of their ways, even deeply bad error, and determine to change their actions and even their whole belief systems, I don't think people change their actual personalities.
I think that a Snape who would personally commit horrific atrocities is not the same Snape who would not stand by and watch death if he could save the person -- even, in the case of trying to save Lupin, a person he disliked, at possible risk of his own life. I could explain in more depth, but I'll leave it at this: a Snape who personally commits serious atrocities is not the same personality as the Snape in canon.
I think that a Snape who would personally commit horrific atrocities is not the same Snape who would not stand by and watch death if he could save the person -- even, in the case of trying to save Lupin, a person he disliked, at possible risk of his own life. I could explain in more depth, but I'll leave it at this: a Snape who personally commits serious atrocities is not the same personality as the Snape in canon.