“Snape, meanwhile, seemed to have decided to act as though Harry were invisible.” (HP/OotP, 660) He avoids eye contact with Harry, in case Voldemort is watching Snape through the scar connection, and behaves spitefully so Voldemort will feel the mutual antipathy between Snape and Harry. This is the easiest thing in the world for Snape to pull off and affords him some nasty enjoyment, but Snape is behaving this way to protect them all. He may enjoy aggravating Harry, but it cannot be argued that these moments of “gloating pleasure” (HP/OotP, 661) interfere with the strategy in any way.
