I like the theory that mcgonagal is a spy but i don't think she is because she is dumbledore's friend....
and do you think that if she really is a spy why would she consider closing hogwarts... We all know that VOLDEMORT consider hogwarts as home so i think he won't let it close.........
back to snape i think his having a revenge on james thru harry... it is really hard to guess if he really is good or bad...... i also think he killedf dumbledore to protect his own skin... i also have the theory that dumbledore only sacrifice himself to save snape's life.. ... to be honest i hate him right from the start