Regarding Pansy - DH SPOILERS
Pansy Parkinson's Notorious Outburst and the Great Slytherin Divide
The divide between Houses existed long before this book. It seemed like it was already in place before Malfoy and Parkinson showed up.
If Pansy had stood up and said, "We're sorry we made fun of Gryffindors, everyone! We'll sacrifice our lives to help you now." I would have thrown my book across the room.
"Potter's there! Someone grab him!" She thinks the Dark Lord will most certainly win, so why should 600 students die fighting for the sake of one boy? Why should she? If they send Harry out there, her boyfriend and friends won't be killed. (Had Draco been there, he would have said the same thing.)
Why should she defend someone she never liked and never liked her in the first place, the same someone who caused her boyfriend lots of hurt (in her eyes, anyway)? Someone who clearly despised her and the House she was sorted into?
Pansy's arm is shaking. She's frantic, and there's not even a question of what to do. She's scared of dying at the hand of the Dark Lord and his Death Eaters, who she had most likely heard frightening things about -- especially about what they might do to witches. (No Parkinsons were ever mentioned as DEs, btw.)
As a Slytherin, she has little faith in Harry's abilities to do anything positive. Some in the other Houses were involved in the DA, so they know a bit about Harry's skills and most likely talked amongst themselves after fifth year. Their families may have been ones that were targeted by the new regime, so they had reason to fight. Maybe they weren't targets but had grown up hearing about and seeing inequities in the wizarding world, like the Weasleys. They had a different perspective because of their families.
Even it Pansy's family didn't subscribe to the new regime's methods, as pure-bloods they weren't being targeted. Keep your head down, don't call attention to yourself. (Sound like the Malfoys in the book?) If there's trouble, come home. Unfortunately, Pansy felt the need to make an outburst, and she never expected that the majority would turn on her, wands drawn.
Slytherin House takes care of its own, and as a seventh year Prefect ETA: Head Girl, she marched her Slytherins out of harm's way. (As did many Ravenclaws, some Hufflepuffs and a few Gryffindors.) Pansy had little reason to believe there would be anything gained and had every reason to believe everything would be lost by staying to fight.
I'm not saying I would have done what Pansy did, or that I condone it. She did was she did because of her social conditioning -- much like Draco. As Draco began to experience the horrors first hand, it began to change his perspective a little bit. He's going to be a more tolerant pure-blood, who still marries pure-blood Pansy and has a pure-blood heir and lives the life they were born into.
I think this experience will have changed everyone to some extent, Pansy included, but it would be unrealistic to expect that there would be a total 360 of Slytherin attitude in the last book. No, that would have had to been something that was nurtured from first year by Snape and Dumbledore, and it wasn't.
ETA: There should have been House unity from day one. You just can't expect something that radical to happen after so much reinforcement by their families and also by the way they were looked upon by their teachers and peers.
Slytherin House is very much misunderstood, and that is the real reason for the divide.
Snape had the same situation when he was in Slytherin, and he also had a lot of baggage about that. I think he did a good job with unifying the Slytherins, but not integrating them. I really can't blame him either.
I think this falls to the Headmaster, and he didn't do much to help. It seemed as if he had a bias for Gryffindor, and it's really a shame that even he couldn't have risen above the established House divide.
ETA: I don't think the Slytherins believed that a few students and teachers could actually stand long against to the Dark Lord and the Death Eaters, much less win. The Slytherins probably knew more about the terror the DEs could inflict and knew if they crossed that line, they were committing suicide. In their perspective, the Dark Lord had already won. He controlled the Ministry, the school and they didn't want to become the outcasts or the hunted in the new regime. No, they weren't going to be foolish and fight against the Dark Lord, even if they really wanted him gone inside.
ETA: And yet another way to say this: The Slytherins were in character, loyal to themselves, not easily swayed and made what appeared to be the right decision to them: turn in Potter and get the hell out of there. They do not know Harry as we do, as the other Gryffindors and some of the Hufflepuffs and Ravenclaws do. They have no faith in Potter or his supposed ability to change the great Darkness that has blanketed their lives. Talk of that is treason! Voldemort's controlling the Ministry, the school and society. The Slytherins are more than frightened: Draco shows his terror and reluctance, and Pansy is on the verge of hysterics as she hears Voldemort's horrifying voice radiating from somewhere previously thought safe. They see no way out. Fighting a lost cause (in their view) is no way to survive. They must be positioned to be favoured in the aftermath. It makes them and theirs stay alive, even if they secretly hope for an end to this madness. The Malfoys illustrate this well. We just cannot put all Slytherins in the same category as Voldemort, Bellatrix and Greyback, and I applaud JKR for showing that in her subtle ways and not giving us a warm-fuzzy.
ETA: Here is a short story about what may have occurred prior to Pansy's outburst.
Surreality ... (Personal Favorite of Author)
Summary: The Dark Lord is coming! A missing scene from Harry Potter and the Deathly Hallows.
Pairing: Draco Malfoy/Pansy Parkinson
Characters: Pansy Parkinson, Draco Malfoy, Minerva McGonagall, Filius Flitwick, Pomona Sprout, Horace Slughorn, Vincent Crabbe, Gregory Goyle, and Severus Snape
Rating/Warnings: R for mild sexual situation (DH Spoilers)
Genre: Romance/Drama
Word Count: 1500
Challenge: Challenge Fifteen: Deathly Hallows - Missing Scenes at romancingwizard
Prompt: Chapter thirty: The Sacking of Severus Snape
Status: Completed in 2007
Canon compliancy: Harry Potter and the Deathly Hallows
LJ Link: Surreality at romancingwizard