It's disorientating, the happiness he feels as watches 'Dear' Aunt Bella throw away all of the pureblood training she's probably been taught, to watch the emotions cycle through her face. Her distress was caused by him announcing that he had banished her off of the Black Family Tapestry as the official Lord Malfoy.
It is a bit petty he admits, as he is talking to her portrait, but Draco decides that it's suitable revenge for the way she treated him during the War, 'punishing' him for the elder Malfoy's inability to serve the Dark Lord. It almost makes him jump around in giddiness when he can finally call her Bellatrix without a dark and very illegal hex being spelled at him via the family Magicks (but Malfoy's don't jump, so he settles for a small grin).