"Snape was willing to let James and Harry die so Lily could liveā" James hung him upside down naked from a tree when he was just a young boy; he doesn't has to be some f*cking saint. š
Also there's no correlation between James and Harry dying and Lily living. They are not some perfectly-safe individuals that Severus is punting into danger to keep Lily alive. He is not hunting down James and Harry to bribe Voldemort into sparing Lily. They are, respectively, a declared and vexatious enemy of Voldemort and the presumptive subject of the prophecy about a child who will have the power to defeat the Dark Lord, and Severus is not in a position to be asking Voldemort for the sun and the moon and the stars; clearly, even asking him to consider sparing Lily was a sufficiently big ask that Severus has no guarantee of Voldemort granting it.
And in all complete honesty, Severus was willing to let James and Harry live so Lily could live: he has committed treason against the most powerful and least forgiving wizard in living memory to give Dumbledore the opportunity to keep all three of them safe, because that happened to be the best way to keep Lily safe.
Read that again: Severus loved Lily so much he took action to protect his longtime tormentor in order to increase the odds of her survival.
Dumbledore's reading of the situation is that of an uncharitable ass.

















