Let's be honest, Henry is really the one who ruined everything. If we want to blame someone, I say blame Henry. Basil says that his catty, bitchy, contrarian, cynical attitude is a put-on, and that he's really a good guy under all of that. That doesn't change the fact that Basil knows his shallow 'beauty is all' philosophy will be a bad influence on the impressionable Dorian, and he was very right.
Basil and Dorian are both at fault in some ways, of course. It's possible this narcissism was an underlying feature of Dorian's makeup. Basil says that he feels like Dorian doesn't love him as much as he loves Dorian, and that Dorian is aware of this:
"I feel, Harry, that I have given away my whole soul to some one who treats it as if it were a flower to put in his coat, a bit of decoration to charm his vanity..."
Henry calls Dorian a "Narcissus," and he's right in the most literal sense as Dorian explicitly in the text "falls in love" with his own image in Basil's paining. And we do see Dorian manipulating Basil's affection for him in some of the earliest scenes of him pouting and insisting Henry stays, a habit which becomes much worse when he's out running around driving people to suicide.
Although, before we as readers even meet Dorian, Henry is already hard at work undermining Basil's feelings for him. Henry insists Dorian is just a stupid bimbo:
"He is some brainless beautiful creature who should be always here in winter when we have no flowers to look at, and always here in summer when we want something to chill our intelligence."
And he also insists that Basil's feelings are purely attraction based on not built on anything real, even though Henry doesn't seem to believe any relationship can be built on anything real:
"Some day you will look at your friend, and he will seem to you to be a little out of drawing, or you won’t like his tone of colour, or something. You will bitterly reproach him in your own heart, and seriously think that he has behaved very badly to you. The next time he calls, you will be perfectly cold and indifferent."
However, I think we can all agree that Dorian's real heel turn came after his solo conversation with Henry. That's when he became obsessed with the painting. Henry starts going off on this kid (he's 21, which is my age, and plenty of guys my age are dumb) saying that he's only good for one thing, and he'll only have that one thing for so long.
"Some day, when you are old and wrinkled and ugly, when thought has seared your forehead with its lines, and passion branded your lips with its hideous fires, you will feel it, you will feel it terribly. Now, wherever you go, you charm the world. Will it always be so?"
It's impossible to deny that before he met Henry, Dorian was a naïve, slightly selfish and airheaded, but all-around good-natured young man. And after he met Henry, he was an actual monster of Gothic horror. However, like in 'Carmilla' and 'Frankenstein' this monster was made. They told him he had power in one way, he had one good quality, and he believed them.
I had a boyfriend freshman year of university who read the first two chapters of this book, got bored, set it down and never looked back. However, he did say: "Everyone says, "Would Dorian," but nobody ever asks, "How's Dorian?" And I think that's a pretty damn important point.