by Lupin » Thursday 9 January 2003 6:07:54pm
The first time Harry saw Snape, Snape was sitting at the professor's table at the front of the Great Hall, and he was sitting next to Quirrel, who had the back of his head pointed toward Harry at the time. Which means that Voldemort's face was pointed at Harry, which is what made Harry's scar burn - it was just coincidental that Snape also happened to be looking at Harry at the same time. This is what led Harry to believe that Snape was against him.
--L