Chivalry. Loyalty. Common sense. Ambition. Justice. Companionship. Order. Self-interest. All excellent qualities, all good reasons to act, all sources of evil.
The Gryffindors joined the Order of the Phoenix to fight for justice. All people were equal and deserved love and acceptance. They saw the atrocities being perpetrated by Voldemort and knew that he had to be stopped. They would lay down their lives if necessary for what they believed was right.
The Slytherins joined the Death Eaters because it suited them. They revelled in the feeling of superiority, and the chance to stamp out blood impurity fulfilled this urge perfectly. It was easier to get ahead when climbing over the corpses of other. They would do what was in their best interests.
The Hufflepuffs joined the Order of the Phoenix to protect their friends. They saw attacks being launched on others that they cared about and loyalty demanded that they interfere. They stood together, an unstoppable force of love and friendship. They would do anything for those they loved.
The Ravenclaws joined the Death Eaters because it was the intelligent thing to do. The wizarding world was overpopulated – even magical resources were ultimately limited. Certain elements of society were less productive than others – the ones who had grown up knowing nothing and had to be educated in every nuance of magic. Things could not continue as they were – logic dictated that order must be restored. They would do what made sense.
The Ravenclaws joined the Order of the Phoenix because it was the intelligent thing to do. The Muggle-born wizards brought fresh blood to the wizarding world, giving it new ideas and an infusion of power. There were too few talented wizards – every one of them was an asset and must be protected. Besides, war would bring chaos – many lives and much knowledge would be lost meaninglessly. Order must be restored. They would do what made sense.
The Hufflepuffs joined the Death Eaters to protect their friends. With every new Muggle-born wizard accepted, several more Muggles found out about a parallel world that they would surely view as a threat. The horrors of Salem were not forgotten. All it would take was a wizard born to a president or a powerful business tycoon, and the wizarding world would be under attack, vastly outnumbered. This could not be allowed; wizards must stick together. They would do anything for those they loved.
The Slytherins joined the Order of the Phoenix because it suited them. The magical world was wonderful – a life of wealth and importance could be achieved fairly easily. Its expansion was opening up more and more dreams and opportunities, things a Slytherin needed like air and water. The subtle balance and beauty of magic was theirs – it would not be destroyed by one man's madness. They would do what was in their best interests.
The Gryffindors joined the Death Eaters to fight for justice. They looked around them and saw Muggles stealing wizards' magic, taking their rightful place. This was wrong – it was not the way things were meant to be. Muggles were different – any attempt to say otherwise was a perversion of reality. They saw the faded jeans and footballs taking over Hogwarts and knew that the Muggles had declared war on their way of life. Something had to be done. They would lay down their lives if necessary for what they believed was right.