"SECTUMSEMPRA!" bellowed Harry from the floor, waving his wand wildly. Blood spurted from Malfoy's face and chest as though he had been slashed with an invisible sword. He staggered backward and collapsed onto the waterlogged floor with a great splash, his wand falling from his limp right hand.
Harry looked at this sight with horror and uttered "no..." escaped his lips a few times. He moved in direction of blood-soaked Malfoy and fell on his knees. His head was spinning, he cried "Episkey" repeatedly, waving his shaking hand over Slytherin body, but it was not enough, blood was still spilling into water covered floor. Malfoy was trembling and started gurgling with his mouth full of blood. Harry couldn't hear anything except for his own blood pounding in ears and muffled... screaming?
Yes, there was Myrtle screaming her guts out "MURDERER, MURDER IN THE BATHROOM". Harry quivered after realisation that she meant him, and looked back at Malfoy, now in shirt completely red, and decided to get help.
He got back on his legs and stumbled out of the ruined bathroom into corridor and yelled "get some help, get a teacher, quick, get a teacher" with great determiation, like his life depended on it. That was quite true, except it wasn't about his life, but- what was maybe even worse for Potter- somebody else's. He heard quickly approaching footsteps and saw person which made this nightmare even worse, coming quickly with robes billowing around him was Snape. Ex-potion master grabbed him harshly without a word, back into the bathroom, and he ran his eyes over the place. When he spotted Draco, he knelt next to him, pulled his wand and waving it tip over wounds, he muttered some incantation which sounded more like humming of a song.
To Harry awe, gashes slowly knitted themselves together, and blood stopped its course down the floor. While Snape was still muttering incantation to healing spell for third time, and Myrtle was sobbing, looming over the scene, Harry was looking at all this from the corner, standing with his mouth a little agape, and reliving the moment when he threw the curse at Malfoy. It wasn't supposed to be like that, he thought, how half-blood prince could write down something horrible like that? Oh Merlin, what will happen to me, to Malfoy?
While Harry stood there, soaked in water and Draco's blood, Snape lifted bloodied boy with a spell and turned in the way to corridor, ranting on his way to the door "You. Potter, stay here and wait for me"
Harry didn't even think about moving, he just stood, watching swirls of red on the water. Thinking about everybody disappointment in him, Dumbledore, Hermione, Lupin, he wouldn't even notice wailing Myrtle, still floating in boy's bathroom. After ten minutes he saw that Snape came back, and closed the door which were open all this time. Teacher only said "go" to the ghost, and she flew into toilet and left, leaving deafening silence.
"Where did you find that spell?" he demanded from dumbstruck boy. Harry slowly looked at him and quietly said "I didn't mean for that to happen, it was supposed to be just a jinx, not... not something like that!" gaining on loudness as he continued. "I don't like Malfoy, he tried to use Unforgivable on me, but then this spell I found... for enemies... and it just clicked, and I used it and..." he ended limply
"For enemies? I won't ask again Potter, stop babbling, where did you find that spell?" snarled Snape.
Harry thought for a second about betraying his source of "potions talents", of book that let him gain Felix Felixis... "but it was half-blood prince who created that spell, it's dark magic!" whispered a little voice at the back of his head."I found it in old Potions textbook" he answered, deciding that it was Snape after all, and he wouldn't give him weapon to use against himself.
Slytherin stared at him, and he dropped his gaze not to look in those black eyes.
Snape must have decided that he didn't need any more information, as he only said "Detention, Potter. Detention every Saturday until the end of term. I'll see you at ten o'clock in the morning, my office""B-but, sir... there's a quidditch match, it's the last-""Ah, yes, there is indeed; poor Gryffindor, not first place this year, shame" smirked Slytherin interrupting Potter's half through the sentence. With that, he spun and left the bathroom.
Harry just stood there, ankle-deep in pinkish water, lost in thought.