To what extent might Stanley be thought a positive force rather than a destructive force in a Streetcar Named Desire?

Although I have little sympathy for Stanley's character, there's a lot to be said for his initial assessment of Blanche and his warnings to Stella about "who" her sister really is. Stanley sees through Blanche from the first moment...... knowing there's something "off" about her behavior, and taking umbrage to her condescending attitudes. That he checks up on her past secretly and unbeknownst to Stella is wrong, but he also attains important information that Stella is either unwilling or unable to deal with; he is most certainly unable to deal, and as a result his own character takes the sinister turn that makes him rather hard comprehend....... and in the end he's just as crazed as his sister in law.

I think that Stanley's character could have been a positive force in other circumstances (had Stella taken his cue from the start), but his latter reactions deem that thought incorrect.

Stanley's Soft Side

Yes, indeed, Stanley, has a softer side. The complete turn-around he pulls in Scene Three from a raging, abusive drunk to a tender, loving husband certainly leaves our heads spinning. “My baby doll’s left me!” he cries, and “breaks into sobs” (3.189). When he and Stella reunite at the bottom of the stairs, it’s a touching and incredibly tender moment. As Stella tells Blanche the next day, “He was as good as a lamb when I came back, and he’s really very, very ashamed of himself” (4.16).

This duality makes Stanley a tough nut to crack. We can't stand him for hitting his wife, then we feel bad for him when Blanche treats him like an ape, and then we hate him when he rapes Blanche.

What’s so interesting is the opposite way these characteristics are interpreted by the two sisters. Blanche makes her opinion pretty clear in a long passage in Scene Four which we distill here for you:

BLANCHE

He acts like an animal, has an animal’s habits! […] There’s even something — sub-human — something not quite to the stage of humanity yet! Yes, something — ape-like about him […] Bearing the raw meat home from the kill in the jungle! […] Maybe he’ll strike you or maybe grunt and kiss you! (4.118)

Wow, Blanche isn't shy about telling us what she thinks. Stella, on the other hand, finds him to be, well, kind of hot. Or, as she says of his violent foreplay, "I was – sort of – thrilled by it” (4.22).