It had kept him awake at night. When he had been away, he had been filled with terror lest other eyes should look upon it. It had brought melancholy across his passions. Its mere memory had marred many moments of joy. It had been like conscience to him. Yes, it had been conscience. He would destroy it…As it had killed the painter, so it would kill the painter’s work, and all that that meant. It would kill the past, and when that was dead, he would be free. It would kill this monstrous soul-life, and without its hideous warnings, he would be at peace.

– Oscar Wilde

The Picture of Dorian Gray, Chapter 20. Dorian picks up the knife with which he killed Basil Hallward and stabs Basil’s portrait of him with it. In destroying the painting he believes that he is killing his conscience and the past and so freeing himself from these. His decision to destroy the portrait is in keeping with his hedonism – to pursue only things that bring pleasure, and get rid of those that don’t.