Whoops, sorry! Bad habit of mine, I automatically assume that everyone in the world thinks the same way I do...

I don't know, it seemed to me in Snape's memories of Lily as if she had tried to talk him out of it in the past, but I can understand her disgust and hurt with his prejudice against Muggleborns. There's only so much she could have done if he was set on becoming a Death Eater.