The pain overtook his body and he fell to his knees. Unable to keep it in he screamed out in pain whilst he gasped for air. The bones in his body shook as if trying to break loose from each other, until it stopped. It hurt, but his body trembled less and he could think clearer, the only reason he knew why he was no longer under the influence of that curse. For a few moments he could catch his breath all the while he tried to check his Punisher's position.
And then it hit again.
The pain flooded through his entire body. His soul continuously tried to break out of his body. He screamed again. Then, as soon as it all had started it ended. He knew he was punished enough, for his Punisher took a seat again.
"You have disappointed me, Severussss," the snake-like man whispered with a threatening edge.
"I have never meant to," Severus hissed between clenched teeth while trying to keep his body under control while the after-shakes lasted.
With a voice so painfully calm, the Punisher said, "liar."
"My lord," Severus whispered hoarsely, but was cut off by his Punisher,
"But it does not really matter," Voldemort spoke, his voice never wavering from that cold, but calm and collected tone he was using. "To think someone like you would betray me. After all I've done to you," a vague attempt to cut in was ignored again by Voldemort as he continued, "I made you master of Potions. Not Hogwarts or some school. You would have become a Potions Master, but not as skilled as with my help…" a sigh of cold and pretended sadness followed all the while Voldemort fingered with his wand boringly, but readily.
"I made you who you are and you betray me. You went against me," stated, still playing with his wand readily, "A fine Potions Master you were," he rose from his seat, "it will be hard to find one who can do better," he circled his pray boringly and then came to a halt right in front of Severus. He kneeled down and lifted Severus's chin with his slender fingers. The pain caused Severus to winch.
"There is one thing I'd like to know, though, before I take away that miserable life you live," Voldemort whispered closely, "what did you think you could gain from all of this? Your freedom?" He laughed loudly when he saw a slight hope in Severus's eyes. "Freedom is power and you've just thrown that away," he removed his hand from under Severus's chin and Severus's head fell to the ground with a thus. It hurt.
"You had a choice. The Malfoy child or Dumbledore. You chose to eliminate the leader of those opposing me," Voldemort laughed at the idiocy, "I thank you," he started walking back and forth in front of Severus, "you gained nothing else. You ruined the hopes of that ridiculous Order of the Phoenix and the brat died anyway, so you saved no one. You failed," Voldemort raised his wand and pointed it at Severus, "did you really think I'd let the Malfoy brat live after Lucius's failure?" A whispered word and a green light later took away the pain.
You might ask the following:
What did Severus do?
Killing Dumbledore was something he had to do to save Dumbledore. Dumbledore knew the Order would do finde without him and did not mind dying for "the cause". Severus contiued as a spy, though, sending messages to the order in secret, never revealing his true identity.
How did Voldemort find out?
Not really important. The point was he found out. You may decide yourself on how.
What happens to the Order?
The Order will win, of course :P
Why did he kill Draco without reason?
According to Book 6 (HBP) Narcissa says something about Voldemort being angry for Lucius failure. He let's Draco make up for his father, knowing he will not succeed. In my eyes I don't think he ever cared for the Malfoy family, or any other supporters, and deals horribly with failures.