If Lord Henry is the Devil's advocate, then Basil Hallward is God's. He's an eternal idealist who truly believes in the innate goodness of mankind.
What he doesn't realize is that he's a good man living in a bad world. He continues to have faith in the possibility of redemption and he's a firm believer in high-minded, pure values like Beauty, Truth, and Love. The problem is, he tends to confuse these things; because Dorian continues to be beautiful, Basil optimistically thinks that he also continues to be truthful and loving:
"Mind you, I don't believe these rumours at all. At least, I can't believe them when I see you. Sin is a thing that writes itself across a man's face. It cannot be concealed. People talk sometimes of secret vices. There are no such things. If a wretched man has a vice, it shows itself in the lines of his mouth, the droop of his eyelids, the moulding of his hands even." (12.8)
Basil refuses to believe that his supposed friends, Henry and Dorian, can be really bad. Ultimately, he pays for his optimism and good faith with the highest price—his life.
As Henry and Dorian drift farther and farther away from him, Basil grows more and more tragic—his truly artistic temperament leads him into worshipping beauty where he sees it (in Dorian), and it clouds his vision. Basil's true problem is that art is more real to him than life. In an artwork, beauty is always a good thing, but in the real world, it's just not.
He doesn't just want life to be like art—he wants life to be art, and vice versa. He believes that Dorian must still have some shred of good in him, if he can continue to look like an angel—and maybe he's right. However, we don't ever get to find out, since Dorian decides rashly to kill Basil rather than pray for forgiveness. Unfortunately, Basil, the pure artist and worshiper of beauty, isn't cut out for this world of evil men, and he loses in the end.