The Slytherin mind — the mind of a true Slytherin, one who wants to be in the house, and chooses to be in the house, and one who belongs there, besides — is not a mind you can cheat. Not really.
Suppose you were to tell it fantastic fairy stories of certain blessed beings who are pure and good and deserve all they have by virtue of their birth, and certain wicked muddy brown things that come in like intruders in the night. Well, one who simply wants to be a Slytherin might believe you, inflamed as they are by a need to be superior, and perhaps also possessed of an adorable childish credulity. Such a being would gladly reduce the world to Mr. Nott’s fables.
But the truly cunning Slytherin would know better. She would become, after some time, too disillusioned for your bedtime stories.
Suppose you were to demand that the Slytherins line themselves all up in rows, heed the call to arms, show the world their stunning power, lie and cheat and steal and kill in the name of some grand cause, toss away even their freedom to demonstrate that their way — the Slytherin way — is better than all the rest. Those who idolize the house and all it stands for would surely be first in line. But those are not the most Slytherin of Slytherins. Not really. Oh, they want to be. But already they prize house and creed above their own survival, and what sort of snake is that?
Suppose you were to present a Slytherin with one path forward — only one. “Here is the road you must take,” you will say. And it leads to a dank and destructive future, a cold cell in Azkaban, certain death for friend and foe alike. It is as unappealing and as likely to sicken one, as horrible, as those midnight swims in the lake the wild-eyed snake-girl prefects dare to take, protected only by Dark and forbidden magic, desperate to bait and kill a merman in the name of house and home and purest blood.
Oh, but the snake-girl prefects are too wild to be true Slytherins. No true Slytherin would take such a plunge. No true Slytherin would destroy themselves in that manner. Someone who rather likes the idea of Slytherin would. But not someone who embodies the house. Given only one terrible path, the true Slytherin would not jump headlong into it. She would carve out a second path. This is the way of the cunning survivalist.
And so it is that Andromeda wakes in the night, and remembers being tugged into the lake on a dare, and remembers almost drowning, and remembers the strong-armed Hufflepuff who pulled her out and wrapped her in his coat, and promised not to tell.
And then her sister, her hair sodden and dark, the merman’s skull in one hand, the shiny P on her chest glinting, coming upon her and saying, “Oh, darling, of course I’m sorry. But don’t you see? If we’d drowned, at least we would have drowned together.”
"Not me," Andromeda had said. "You drown. Not me."