Over the Weekend Rob Knox, who plays student 'Marcus Melby' in the upcoming 'Harry Potter and the Half-Blood Prince', was stabbed to death outside of a bar in London. Rob was trying to protect his younger brother from a man with knives when he was killed. Rob's friends called him a "Very thoughful Person, who was generous with his money!".. Rob was only 18 years old...A statement from Warner Brothers "We are all shocked and saddened by this news and at this time our sympathies are with his family!" CLICK FOR NEWS VIDEO!!!