Let’s take Dumbledore’s word and believe that Snape has come back to the good side. After the fall of Voldy, Snape comes back to the Order. Dumbledore testifies on his behalf in front of the Wizengamot and tells them that, at great risk to his own life, Snape turns spy against the dark side.
And we know from reading that it was Snape who overheard the Prophecy and told Voldemort. Then once Lily and James die, he realizes what a mistake he made and comes back.
So, if we take all of that… Does that mean that Snape knew that Wormtail was the traitor? And if so, when Sirius was locked in jail for 13 years, doesn’t that mean that Snape knew Sirius was charged incorrectly and did nothing about it? That doesn’t seem like the workings of a good guy.
Don’t get me wrong, I fully believe that Snape is actually working for the Order and doing everything under the orders of Dumbledore. But in book 3, he seems very determined to put Sirius away. For someone who is on such close terms with the Dark Lord, he must have known what the plan was that night, right?