Ok this is a little thought after rereading Goblet of Fire.
It's under a Lj cut as well one, I do not want to inflict my random thoughts on you and two, it contains HBP spoilers
If Snape is guilty why did he show Fudge his Dark Mark in GoF?
Snape strode forward, past Dumbledore, pulling up the left sleeve of his robes as he went. He stuck out his forearm and showed it to Fudge, who recoiled.
"There," said Snape harshly. "There. The Dark Mark. It is not as clear as it was an hour ago, when it burned black, but you can still see it. Every Death Eater had the sign burned into him by the Dark Lord." (GoF 36)
Why would he have revealed himself to Fudge like that?? Dumbledore did not direct him to do it he did on his own; it strikes me as odd that he would reveal something as important as that if he was not backing Dumbledore all the way.
"Severus," said Dumbledore, turning to Snape, "you know what I must ask you to do. If you are ready ... if you are prepared ..."
"I am," said Snape.
He looked slightly paler than usual, and his cold, black eyes glittered strangely. (GoF 36)
This indicates to me that there is already a plan in action. In HBP Snape says
In Spinners end
"Think!" said Snape, impatient again. "Think! By waiting two hours, just two hours, I ensured that I could remain at Hogwarts as a spy! By allowing Dumbledore to think that I was only returning to the Dark Lord's side because I was ordered to, I have been able to pass information on Dumbledore and the Order of the Phoenix ever since! Consider, Bellatrix: The Dark Mark had been growing stronger for months. I knew he must be about to return, all the Death Eaters knew! I had plenty of time to think about what I wanted to do, to plan my next move, to escape like Karkaroff, didn't I?
"The Dark Lord's initial displeasure at my lateness vanished entirely, I assure you, when I explained that I remained faithful, although Dumbledore thought I was his man. Yes, the Dark Lord thought that I had left him forever, but he was wrong."
Is it me or does this seem contradictory?
I mean It seems Dumbledore knew this was coming in GoF he knew even Hagrid knew Voldemort was coming back I believe Severus is lying to Bellatrix here. Sirius says in the beginning if Dumbledore was bringing Moody in to teach he was reading the signs. So Dumbledore knew Voldemort was planning to return and they had Trelawney prediction, Snape knew Voldemort was coming back and he and Dumbledore had a chance to set a plan into motion then.
There is lots of evidence to support that Severus Snape is still working for the good side.
And it’s not my position to pull it all out each person has there own beliefs and theories
What ever I will leave you clear with mine
I trust Severus Snape