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 :)