Depends upon your definition of evil, I suppose. He betrayed his chosen leader for personal greed. He didn't do what I demanded, therefore I will work against him. He had no trouble with children and adults being murdered so long as it wasn't the woman he had taken a fancy to and been rebuffed by.
I see an explanation for his betrayal, not that he was 'good' inside. He did it for his own self satisfaction, not because it was the right thing to do.
That's my nephew's take on Snapes.
I think I was pulling for Snapes mostly because the actor's portrayal in the movies is such genius.
Snape's love for Lily was THE most powerful influence in his life. It consumed him. Dumbledore understood this, which is why he trusted Snape to do the right thing when the time came.
He knew Snape would help to defeat the wizard who had killed his love ... and who, not incidentally, had killed Snape's chances of gaining Lily's love. His loyalty to Voldemort was acting when he called her a 'Mudblood.' He lost the one good thing in his life to Voldemort.
Voldemort couldn't read Snape, because he couldn't get past Snape's love of Lily. Everything about Snape's behavior can be explained by referring back to his love for her.