Geez, Draco pipe down. Since when did you come over all hearts and flowers for a Gryffindor of all witches? He put the poisoned wine in Slughorn's office, and waited for Dumbledore to drink it. Yes, he knew that had to work. It had to, because he certainly did not want to go to Azkaban like his dad. His dad was still haunted by what happened to him while behind bars.