The first rule that Slytherins tend to learn is to hide their emotions and to lie whenever, something that truly matters (like the truth) that can easily manipulate that person.

Severus Snape might have lied to Albus Dumbledore that day.

Professor Snape might be one of the meanest, nastiest professor to Harry Potter. But he has a heart and he really does care about Harry.

So maybe at the end of the day not everything he does for Lily. He does it to protect Harry and make sure that in the end that Harry doesn't die.

He does everything cause he does care about Harry Potter.

And guess what? Albus Dumbledore knows that.

.

And someday, maybe, Harry Potter will know that as well.