Dorian begins the novel as an innocent youth. Under Lord Henry's influence he becomes corrupt, and eventually begins corrupting other youths himself. One of the major philosophical questions raised by this novel is that of where to locate the responsibility for a person's misdeeds. If one engages in a moralistic reading, The Picture of Dorian Gray can be seen as a lesson in taking responsibility for one's actions. Dorian often points to Lord Henry as the source of his corruption. However, when contemplating the plights of others, Dorian lays the blame at their own feet rather than considering the role that he might have played in their downfall.
As to who is really to blame, this calls for your opinion. I believe that we are all morally responsible for our own choices. We can choose to act individually, or we can choose to follow the crowd. We can walk away.... or we can participate. In your opinion, did Dorian and Basil have choices in the paths they followed. Did they choose? Were they forced? Therein, you will find your answer to the second part of your question.