In Oscar Wilde's novel The Picture of Dorian Gray, we can go back as early as chapter 2, when Lord Henry Wooton meets Dorian for the first time, to determine the moment when Wooton's influence on Dorian begins to take effect. We now that, from the moment he sees Dorian, Wooton is determined to begin his agenda of inviting Dorian to use his own youth and beauty to "explore all the senses", and to go against the grain in terms of all things good and decent.
We know that Dorian Gray first takes these words like a grain of salt and is even shocked to hear someone say that being good, virtuous, or even charitable, is actually boring and silly. That, when one is as good looking and young as Dorian, the best thing to do is to exploit those gifts in a world full of temptations so that one can really learn the meaning of "living".
For nearly ten minutes he stood there, motionless, with parted lips, and eyes strangely bright. He was dimly conscious that entirely fresh influences were at work within him. Yet they seemed to him to have come really from himself.
However, there is a subtle message sent implying that Dorian has not discovered anything new. That, instead, Harry is actually pushing Dorian to identify that part of himself which is already pagan and hedonistic.
The few words that Basil's friend had said to him—words spoken by chance, no doubt, and with wilful paradox in them—had touched some secret chord that had never been touched before, but that he felt was now vibrating and throbbing to curious pulses.
But the absolute instance when Dorian makes his complete turnaround comes in chapter 8, after Dorian finds out about the suicide of his former fiancee Sybil Vane hours after he abandons her coldheartedly for being a bad actress. It is curious to Dorian that, after finding out the news, he feels no grief whatsoever. This, of course, is partly influenced by Harry who is the bearer of the sad news. However, it is Dorian, on his own, who admits that his grief for Sybil's death has actually uncovered the Pandora's box that is Dorian's soul: A dark and crazy place where only that which is forbidden is allowed to take place:
He felt that the time had really come for making his choice. Or had his choice already been made? Yes, life had decided that for him—life, and his own infinite curiosity about life. Eternal youth, infinite passion, pleasures subtle and secret, wild joys and wilder sins—he was to have all these things. The portrait was to bear the burden of his shame; that was all.
It is after this moment that Dorian Gray begins his rampage throughout London, focusing only on materialistic things, exploring the underbelly of the East end, dabbling in opium and alcohol, and ruining the reputation of every man and woman who come in contact with him. While all this happens, Dorian's looks remain young and angelical while his portrait, which represents his soul, acquires the horrid looks of sin and evil that Dorian brings to it.
In the end, we will see that Dorian destroys the picture and dies as a result of it. His body will turn into a that of a monstrous looking old man, while the picture returns to its original beauty.
Just a thought- his quest for the perfect life, beginning with his weakness for vanity is one of the things which leads to him lose his concept of morality.