If you haven't read Rex Luscus's great essay, "Snape's Supposed Great Love," do so! S/he looks at Severus's Patronus as a metaphor, and points out the canon evidence for Severus's spiritual growth: from turning from the DE's (solely) out of selfish/acquisitive desire for Lily, to saving Lily's son because it's what she would have wanted, to sacrificing everything to save others, because it's the right thing to do.
Snape may have started by rejecting Voldemort only because of the threat to Lily, but even by the time of her death that was no longer his sole motivation. Otherwise, if all he cared about was possessing Lily and increasing his own power/promoting an anti-Muggle agenda/whatever reason you think he joined the DE's, with Lily lost he'd turn back, if not to the DE's (tactically imprudent), to whatever he thought could achieve his secondary aims (power or whatever). And he doesn't; he continues to do "anything" DD asks of him, to protect Lily's son.
Turning from the Death Eaters
Snape may have started by rejecting Voldemort only because of the threat to Lily, but even by the time of her death that was no longer his sole motivation. Otherwise, if all he cared about was possessing Lily and increasing his own power/promoting an anti-Muggle agenda/whatever reason you think he joined the DE's, with Lily lost he'd turn back, if not to the DE's (tactically imprudent), to whatever he thought could achieve his secondary aims (power or whatever). And he doesn't; he continues to do "anything" DD asks of him, to protect Lily's son.