Slytherins truly being slytherins.
Slytherins working with Dumbledore’s Army. They spread false rumors about where they’ve sighted students. They join Umbridge’s close circle so that they can learn the moves and plans of the Inquisitorial Squad so that they can warn the DA of when it’s safe to hold practice. They rotate in shifts, some lead the IS on a wild goose chase while their housemates practice with the rest of the army. Sometimes they charm their robes to look like another houses and lead the IS far away from the Room of Requirement as they can before turning a corner and de-charming their robes. They make up stories about chasing students who mysteriously disappear without a trace. Umbridge sends students to search areas of the castle nowhere near the actual Room of Requirement.
As Umbridge tightens her hold on the school, slytherin students try to protect their DA friends. They pass messages of encouragement and support to their friends, veiled in threats and taunts. They bully the loudest, but do the least damage; their housemates think that they’re being cruel, their friends know that they’re shielding them from actual harm.
Slytherin students passing information to the Order while the Golden Trio is hunting for horcruxes, tracking the moves of Death Eaters to the best of their ability. They withstand the dark days in Hogwarts, helping their friends the best they can without looking suspicious. Charmed plastic rats run through the castle hallway, passing messages between the houses. Children of Death Eaters eavesdrop in dark hallways of their homes, holding their breath and straining their ears. When slytherin students suddenly disappear, go home and don’t come back, everyone knew what had happened. They mourned privately and quietly.
DA members and the Order housing slytherin students who can’t go home. Many have to run away, for their own safety. As Voldemort grew in power, it was harder and harder to keep their true intentions a secret. Children of Death Eaters were being pressured to take the Dark Mark, were being drafted to fight for Voldemort, were being told that their friends deserved to die. The students that said no were threatened with death or kicked out of their homes, branded as disgraced traitors. Those who couldn’t stand up to their families slipped away in the night. Some died, at the hands of Death Eaters, at the hands of their own parents.
Slytherin students fighting during the final battle at Hogwarts, standing up against family and friends to protect their home. Staring across the courtyard at their mothers, fathers, siblings, childhood friends. People screaming at them; ‘blood traitors’ ringing through the air of the courtyard, hurled at slytherins who had their shoulders squared and their chins up. Slytherins having to throw charms and jinxes and curses at familiar faces, people they had trusted. Slytherin students killing their loved ones, being given no other choice. Slytherin students dying while standing up for what they believe in.
Slytherin students being more than a stereotype. Slytherins being more than one dimensional villains. Slytherin students being shown as diverse, fully developed characters with a variety of motivations and beliefs. Slytherins being slytherins; proud, ambitious, and cunning. And proving that those things don’t mean being evil.