Im rewatching harry potter and the half prince... I realized that Severus originally was a death eater, then became a double agent for albus during the first wizarding war, then when Voldemort returned he convinced them he was a triple agent and was actually always working for Voldemort... but in fact he was a quadruple agent and was, in fact working for the order of the phoenix all along.

I love harry potter and all but man JK Rowling was not the best at realism :)

@freemo I think it was meant to be taken as he really did side with the death eaters, but gave it up at great risk for love.

@obi Well yes, originally in the first wizarding world whenn he first joined the death eaters it was sincere... but then (for love) he became a double agent... then he became a triple agent... and thenn we learn he was really a quadruple agent all along :)

@freemo I guess I see it as he is a death eater, even when being a double and quadruple agent, and his love for Lilly was so strong that he betrayed his kind. Interesting to think about his motivations though, if there was more.