The song sung by Kurt is "This is the Moment" from Jekyll and Hyde. You can listen to it here tiny. cc/ a74ko (youtube link)
They didn't find him. Finn sat at the back of the choir room, staring blankly down at the back of Kurt's head. Alex had run off in the night last week, and they had searched all day, calling his name, looking everywhere, but they couldn't find him alive or dead. Burt had called both Kurt and Finn in sick, and they skipped school for the rest of the week, searching for Alex, reading newspapers and watching the news for reports of a body being found, and stumbling around the house in blind dazes. They were back at school now, at Glee, for the first time since Alex's disappearance. His death. Kurt pointed out quietly that if they didn't find him that first morning, chances were very, very good that he was well and truly dead. I'd never kill myself, Kurt had whispered, reaching over to take Burt's hand, too much to live for, but if I did... if I said I would, then I would. I wouldn't back out. If things got that far...
Kurt had been the one holding them together these past few days. He had been the one remembering to eat meals, forcing them to sit down, to talk. He had been the one who had slowly eased them out of their grief, enabling Burt and Finn's mom to return to work and Finn and Kurt to go back to school.
And now he was getting up to sing a song for his Glee assignment. He looked a little pale and shaky, but he was blaming it on bullies. No one else in Glee knew that Finn and Kurt had lost their brother last week.
Kurt met Finn's eyes and smiled weakly before he squared his shoulders, lifted his head, and started to sing.
This is the moment!
This is the day,
When I send all my doubts and demons
On their way!
I have made - ever -
Is coming into play,
Is here and now - today!
This is the moment,
This is the time,
When the momentum and the moment
Are in rhyme!
Give me this moment -
This precious chance -
I'll gather up my past
And make some sense at last!
This is the moment,
When all I've done -
All the dreaming,
Scheming and screaming,
This is the day -
See it sparkle and shine,
When all I've lived for
For all these years,
I've faced the world alone,
And now the time has come
To prove to them
I've made it on my own!
This is the moment -
My final test -
I never reckoned,
I won't look down,
I must not fall!
This is the moment,
The sweetest moment of them all!
This is the moment!
Damn all the odds!
This day, or never,
I'll sit forever
With the gods!
When I look back,
I will always recall,
Moment for moment,
This was the moment,
The greatest moment
Of them all!
The song started off slow and subdued, but Kurt hit his stride as the music crescendoed, belting out the words with a triumphant smile. He had his arms stretched out before him, drinking up the cheers and applause from the entire club. From the entire club... except Finn.
Finn couldn't breathe. He could only stare at Kurt, could only watch as Kurt lifted a hand, flicking his bangs into place a self-satisfied smirk on his face. That wasn't the expression of a boy who'd lost his brother, no matter how much Kurt loved to sing. That was... that was the expression of a boy who'd found a life.
He had to get out. Had to get out. Finn stumbled to his feet, knocking his chair over in his haste to dive toward the door. He could hear Rachel calling after him, the worried voices of the Glee Club following, but he couldn't, he couldn't, had to get out, get out get out get out...
Finn ended up huddling in the locker room showers, curled in a corner of one of the stalls, staring at the tiles and shaking. Footsteps. He could hear footsteps. Someone approaching. Finn turned his head to the side when the steps stopped, staring at a pair of very familiar Doc Martins, Kurt's favorite boots. Only those weren't Kurt's feet. Finn trailed his eyes up and up until he was looking at his stepbrother's face.
At Alex's face.
"Where is he?" Finn asked hoarsely. "What did you do with Kurt?"
"I am Kurt," the other boy answered smoothly. "Finn, come on. Everyone's worried about you." He dropped into a crouch, graceful as ever, holding out one hand to help Finn up.
Alex and Kurt were identical. They were practically the same height (only when they stood side-by-side now could anyone tell Kurt was half an inch taller), and Alex no longer looked gaunt and haunted. They wore the same clothes, styled their hair the same way. They moved in unison, spoke in unison, existed in unison. There was no way Finn could prove that the boy in front of him was one or the other.
But he knew. He knew. Finn looked into those eyes, cold as ice, and he knew this wasn't his Kurt. "No," he said, taking the other's hand and squeezing it until the other boy winced. "No. Tell me. Tell me where Kurt is!"
"Finn, stop it!" he hissed, yanking his hand away. "I'm Kurt. There's only room for one Kurt!"
"No, there isn't! There could have been two! You could have both lived!" Finn shook his head desperately. "Alex, please, where's Kurt? What did you do with him? Please!"
"What are you going to do?" Alex demanded, straightening up. "Tell Burt? Call the police? What are you going to tell them? That I'm not Kurt Hummel? Any DNA test will prove you wrong..."
"What did you do with Kurt?"
"Why does it matter which of us is here?" Alex folded his arms, scowling at Finn. "Aren't we the same person?"
"Kurt would never have kicked you out!"
"Kurt was squandering his life!" Alex snapped. "Sulking around, thinking it was so much worse than it actually is. So he's bullied a bit. So what? He had no idea how good he had it! I'm actually appreciating his blessings!"
"Where is he?" Finn whispered.
Alex lifted his chin, looking off to the side. "You won't find him. I buried him in a shallow grave in the woods, so the animals will find him first." His eyes slid back to Finn's, cold, hard, threatening. "And if you tell Burt or Carole? What good would that do? Burt thinks I'm his son. He loves his son. If you tell him Kurt's actually dead and Alex is in his place? You'd be lucky if he just had a heart attack. You'd kill him, Finn. His death would be on your hands. And why tell Carole? She'd only tell Burt, and the same thing would happen. And no one else would believe you. Alternate universes? The 'evil' twin?"
"You killed Kurt...?"
Alex snapped his fingers in front of Finn's glazed-over eyes. "Suck it up. You never liked him anyway. Too gay. But I'm here now. And I know all about Kurt's life, his friends, his Cheerios routines... I'll fit into his life, and don't you worry, Finn. Unlike Kurt, I can actually kick some butt. The bullies will back off. Not just me, they'll leave you alone too. Trust me, Finn." Alex took a step back, smirking. "It gets so much better..."