The colour seemed to drain from Snape's face as he heard this. A slight tremor ran through his hand as he raised his wand at the beast which was almost upon them. "Mister Potter, I would recommend looking away." Harry, confused at Snape's words, chose to ignore him completely. "Fine. Avada Kedavra." A flash of green light flew from his wand, it hit the troll with very little impact yet as soon as it made contact with the beast, the troll crumpled, as if the strings which had been holding it up were cut.