How does Stanley's behavior lead to Blanche's downfall in A Streetcar Named Desire?

Stanley is fed up with Blanche's presence and the way it has disrupted his life with Stella in their tiny apartment. He also resents the airs that Blanche puts on.

Stanley investigates Blanche's past in Laurel and discovers she has been lying about her life. She lost Belle Reve long ago, not recently, and moved into a seedy hotel called the Flamingo, where it is implied she worked as a prostitute before being asked to leave. Blanche lost her teaching job because she had an affair with a seventeen-year-old old student. By the time she got to New Orleans she was desperate and out of options. Nevertheless, Stanley cruelly gives her a bus ticket as a birthday gift, even though he knows she has nowhere to go.

Stanley also tells Mitch all about Blanche's sordid history, saying he has to protect his friend. Mitch gives up any idea of marrying Blanche, which was her last hope of salvaging a life, yet wants to have sex with her. Blanche protects herself by screaming "fire" but is not so lucky when Stanley comes home while Stella is in the hospital giving birth. Stanley rapes Blanche, causing her fragile mind to finally snap. At the end of the play, she is taken off to an asylum.

Stanley's crude brutality destroys the fragile, romantic Blanche, caught up in dreams of the past.

How is Stanley unnecessarily cruel to Blanche in A Streetcar Named Desire?

In Tennessee William's play A Streetcar Named Desire the character of Blanche Dubois, a Southern belle of sophisticated tastes and mannerisms and formerly rich woman, enters the household of her sister Stella and her husband Stan Kowalski with the alleged purpose of visiting and spending time with her sister. Soon after she sets foot in the house, the problems begin.

The main problem between Stanley and Blanche is that both are quite set in their ways, and none of them has a trace of similarity with each other's upbringings. Stanley is a rogue, rough, and chauvinistic Polish American man whose salient traits are his foul language, his harsh treatment of all woman, and his love for gambling and drinking.

Contrastingly, Blanche is a woman who has seen much better days but whose reality has changed so drastically that she cannot fathom the fact that she is now a destitute and a social outcast. The only way that Blanche can do to save some face is to revert to her conduct back to when she was a proper Southern belle whose only worries were to look pretty and be liked. However, she tries too hard to be liked by someone by Stanley who, on principle, does not appreciate imprudence, particularly if it comes from a female.

Hence, Stanley makes it clear to Blanche that she is not welcome nor is she going to make his household her headquarters for change of life. As a result, he consistently insults both Stella and Blanche and questions her past as to get to the real reason why Blanche would have come to visit and stay with them in the first place.

The worse act of cruelty comes at the end when Stanley completely breaks the woman that is Blanche both inside and outside. From the inside, Stanley breaks her dignity and respect by investigating everything about Blanche and by openly calling her on her lies about her past. To top it all, he goes to Mitch, the man in which Blanche has put her last hope  to make a better life, and tells him all about, prompting Mitch to abandon Blanche- and all her hopes.

To make matters even worse, Stanley literally violates Blanche sexually and attacks her in such a way that she has to be placed in an asylum. This, with the complete cooperation of her sister who, in her own denial, pretends that nothing happens and continues to coexist with her abusive husband, giving more importance to him than to her own blood.

Therefore, Stanley rips the pride off Blanche by exposing her for who she really is, not caring whether Blanche is seeking for a way to become a better person. He takes away her chances with Mitch, which means that it will be nearly impossible for Blanche to see for a better future in that town with a bad reputation. Finally, he rapes Blanche as an ultimate act of chauvinism and disrespect: This finally destroy all that Blanche is and it is, indeed, done viciously and unnecessarily.