What is the ruling idea of A Streetcar Named Desire?

Probably the title says it all in this play. Williams focuses on the power of "desire," not only in a sexual sense, but as a ruling force by which the behavior of people is governed—often, if not primarily, to their own detriment.
It's hard to avoid asking why the collection of characters in Streetcar act as they do. The only answer is that self-destructive desire is an element of "human nature." The relationship between Stella and Stanley is dysfunctional, governed by his abuse of her. At first sight they would seem to be mismatched, given that Stella is from a supposedly genteel background which her sister, Blanche, still cherishes and believes in. But the implication is that Stanley's crudeness is his principal attraction for both sisters, despite Blanche's open contempt for Stanley. The concept of desire is related to the fact of human beings needing others who represent fulfillment for them, although these needs are what often end up victimizing them.
Blanche could conceivably have simply ignored Stanley. She is attracted to Mitch as well—in a different, more "respectable" way—but her smoldering tension with Stanley is what indirectly leads to Mitch's turning against her. Stanley's investigation into Blanche's "past" is a form of revenge against her over her obvious contempt for him, and it ruins whatever happiness she could have had with Mitch. All in all, it is the dark side of "desire" that seems to dominate all the characters, perhaps even Mitch.
Even at the start of the play, Blanche is already a woman half-destroyed by her own needs. Tormented by guilt over the suicide of a man she was in love with, she seems primed for an act that will complete a cycle of self-immolation. The "streetcar" she has taken to the home of Stanley is one by which her own desires lead to her ultimate victimization, in the tragic conclusion.
This is not to imply that the outcome is Blanche's fault. To say this would be a classic "blaming the victim" scenario. Rather, Williams's message seems more to indicate a randomness about the outcome of human interaction. People cannot control their needs, their deepest desires, and the unfortunate fact is that anyone can end up suffering the largely unpredictable results of the primal forces that animate us.