Tom acts like an animal when he’s angry. The usual Tom goes some place else and something else takes over.
It’s the word mudblood that does it.
Before, Tom had been in a good mood. It was his first day at Hogwarts and he had experiences that he was cradling close, holding lovingly in his mind. His first view of Hogwarts- the huge dark castle against a deep blue sky, windows a warm inviting yellow, the way the Sorting Hat had whispered, ‘Welcome home’, the luxury of the start of term feast...
A part of Tom holds back from everything, pretending not to care, trying to protect itself from the misery when all this will ultimately be taken away. Another part of Tom indulges, like a snake luxuriating in warmth, he opens himself up to this new world, allows it to wash away all the misery and meanness of the previous one.
But now he is in his dorm, reading while reclining on his bed, and hears that word, mudblood. His ears prick at the familiar contempt flung upon him, the part that pretends not to care is the same part that flips the switch inside his head.
He acts. Later, he’ll piece together what he does: he drops the book in his hand, he turns slowly to the white haired boy-Malfoy- who said the word. Tom's eyes are burning, his jaw clenches. He stares for a moment, the word echoing in his head, and then he leaps.
Malfoy has clearly never been in a fistfight.
He doesn’t raise his fists, he doesn’t back track. He just stands, rooted like an old tree, and collapses under Tom’s weight when Tom collides into him. Tom is a lean, thin little boy but he’s been fighting before he started speaking. He makes sure to cushion his own fall using Malfoy’s body and in the process, drives the breath out of Malfoy’s lungs.
Then he raises his fist and rams it into Malfoy's face.
His other dorm mates, Avery and Lestrange, have clearly also never been in fist-fights, because Tom lands three very satisfying punches, before they unfreeze and think to intervene.
Avery gets an electric shock when he puts his hand to Tom’s shoulder. He screams, tearing his hand away, and backs off.
But Lestrange simply raises his wand and sends Tom flying into the air.
Tom has just a second to widen his eyes in surprise, then narrow in rage at Lestrange’s expression, before he crashes into the mirror behind him. The mirror shatters, raining shards over an out-of-breath Tom.
For a moment, there’s silence. Then Tom raises his head.
He doesn’t know it yet, but the mirror shards have cut him, and blood is welling up on his arms, his face, his neck. He’s also managed to cut his lip on his teeth. So the smile he directs at Lestrange is bloody.
Then he raises his hand, and Lestrange comes flying to him. Tom has a moment to enjoy his horror and shock, and then he shifts and turns, pinning Lestrange to the wall with a hand to his throat. He finds a shard of glass with his other hand, and turns to look at it. It’s sharp enough. He raises the glass to Lestrange’s throat.
Lestrange starts quivering.
“I can cut your throat with this,” Tom whispers.
He moves the shard higher so the point is under Lestrange’s eye, “Or should I stab it into your eye?”
“No, please,” Lestrange whispers.
Tom waits for a long moment, then continues, “But since you don’t know the rules yet, I’ll be merciful.”
He moves the shard to Lestrange’s cheek and presses the tip in.
Lestrange screams and writhes, but Tom holds him still, and cuts a careful deep line from corner of eye to jaw.
He looks up just as Avery moves to the door and the other boy freezes in his place.
Tom stands slowly and walks towards Avery.
Avery backs away until he’s pressed against the door.
Tom leans in close, enjoying the terror on Avery’s face, and says loud enough for the other boys to hear, “Don’t forget we’ll share this dorm for many years.”
Tom leans back, then waits until Avery nods jerkily, avoiding his eyes.
He walks back to his bed, picks up Hogwarts, A History, reclines back against the luxurious cushions and continues reading.