What are the characteristics of Dorian Gray that lead to his downfall in The Picture of Dorian Gray?

Fundamentally, Dorian is narcissistic, vain, and selfish.
He is obsessed with beauty, and sensual experience to the point that he has no balance
in his life. He is so focused on fulfilling his sensual desires that he loses touch with
reality. While these characteristics do intensify as the novel progresses they are
evident even when we first meet him at age 20.

The first
direct sign of Dorian's downfall is his unsympathetic response to Sybil's suicide. The
fleeting nature of his love/lust for her suggests that he is imbalanced. Furthermore,
this sign, if not initially recognized by the reader, is confirmed when it represented
as the first change in the portrait.

As his obsession with
beauty, vanity, and himself intensifies, Dorian becomes more mentally unstable,
eventually resulting in him killing Basil. From this point on, the reader can see
Dorian's descent into madness is inevitably going to result in his
demise.