Disclaimer: The characters, curses, Hogwarts, etc. all belong to J.K. Rowling and her publishers. The plot of most of this, except for that Voldemort returned to power, belongs to me. I'm not making any money off of this. Don't sue me.
As Harry plucked one petal of a rose and it fell onto her grave, he thought, "It's all my fault she's not alive. My fault she lies in here. I should be in Azkaban right now."
He thought, "She had only been sixteen. I was just one year younger. How could I have done this to a peer? One who shared my joys, and shared my sorrows, one who I loved? How could I have killed her so easily? Why did I have to lose all trust in everyone?"
Harry remembered, "I thought our love was thick. I thought our bond was strong. That we couldn't break it. That no one could. But only when it had already been broken, damaged beyond repair, did I realize the truth. That a few words uttered from her lips could make me forget our love. Forget everything, in the face of a few words."
"Why was I so stupid?" His lips formed the words, but his throat could not speak.
Fifth Year
It was during Harry's fifth year, Cho's sixth. Voldemort's power continued to ascend, to heights which no one thought was possible. It was a time when the era of fear, of mistrust, which Harry had heard of, returned.
No one knew whom to trust. Whom was on their side. If telling someone could save your life, or cost it. Could you confide in the girl sitting next to you? Or had she taken part in the killing of your parents?
But Harry thought that he trusted some people. Some people, he would only believe had gone over to the Dark Side if Voldemort confirmed it. Like Harry. Like Ron. Like Dumbledore. Like Cho.
He had been wrong.
Adulthood
Another petal fell from the rose and fluttered gently onto her grave. "Cho, how could I have done that to you?" Harry asked himself silently. "How could I have been so stupid? Why did I let myself be pulled into that trap?"
"It was what Voldemort wanted," Harry thought bitterly. "He loved the mistrust. He knew it planted seeds of discord. Seeds of death. Seeds that, though they seemed harmless at first, would later have terrible consequences."
"I had done Voldemort's bidding," Harry thought, as another tear stung his eye. "Voldemort wanted me to do it. Maybe he couldn't make me go over to his side. But his power still made me kill Cho. He still made me lose the one I love."
"I hate what you made me do," Harry silently told Voldemort, though he knew Voldemort couldn't hear.
Fifth Year
Cho and Harry loved each other. They had a bond they thought nobody could break. Harry didn't replace Cedric for Cho, but she still loved him, for she could love after death. They knew each other so well, and thought it was one thing Voldemort couldn't shatter. Until the dance.
Cho and Harry had been dancing, when Cho whispered into his ear, "Harry, there's something I need to tell you." They both moved over to an isolated corner, as Harry wondered what it could be.
"Harry, I need to tell you this. Please don't overreact. It's going to be shocking to you at first. My parents went over to Voldemort, but I --" Cho began to speak, but Harry cut her off in a flurry of movement.
Harry couldn't think. The only thought in his mind was that Cho's parents had gone to the Dark Side, and that she would be on that side, too. That she wanted him dead. She was his enemy, one not to be trusted, not to be loved, only to be feared and to be hated.
Pointing his wand at her, the lover-turned-enemy, he whispered the words, barely audible to even himself: "Avada Kedavra." The Death Curse. An Unforgivable Curse. Cho fell over, lifeless.
Adulthood
"Why," Harry wondered bitterly, as he let another petal fall, "could I just not listen to her? Just let her speak for but a few seconds. It would have been a matter of seconds that she would have needed, and she would have survived."
"Of course," Harry told himself, "it's easy to say that now. Now that Voldemort is defeated. Now that I can trust others. Now that I don't have to worry that your best friend could be helping your parents' killer. Now that you don't think that one slip of the tongue to a person could not cost your life."
Harry knew he'd always been impulsive. Always done things fast, not thinking about them like Hermione did. He remembered Hermione's words after it all: "Oh Harry, why couldn't you just have listened to her? Why couldn't you have just waited a few seconds, so that you could think about what to do next?"
But he hadn't thought. He had gone on his impulses, and it had cost Cho her life.
Fifth Year
What happened next seemed more of a blur than actual life, as if everything was on fast forward. Harry remembered looking done at Cho, wondering what would happen now that he had used an Unforgivable Curse, even if it was on a person who supported Voldemort.
But, Harry was soon to realize, she didn't support him. Moments later, he saw Hermione's face, felt her hand touching his arm. "She told you about her parents, didn't she?" she said, knowing already what had happened.
Harry looked up at her in shock. "You knew?" he said in disbelief. "You knew she was on the Dark Side, and you didn't tell anyone?"
A tear escaped Hermione's eye. "Oh, Harry," she said. "But she wasn't. She told me already. Her parents had joined Voldemort. But she was still on our side. She hadn't joined him. She was all right." Hermione began to sob.
Harry put his hand to Cho's lifeless body. "She was...she was innocent?" he asked Hermione. "I just killed an innocent person?" He was in a shock beyond any shock he'd ever known.
"Y-yes," Hermione sobbed. "Oh Harry, why couldn't you just have listened to her? Why couldn't you have just waited a few seconds, so that you could think about what to do next?"
But it was too late. For what Harry had done, there was no counter-curse. Cho was dead, dead forever, because of Harry.
Adulthood
Harry watched through his tears as another rose petal fell. "I should have gone to Azkaban for life. That's what's supposed to happen to someone who uses an Unforgivable Curse on someone."
"It's what I deserved," he thought. "For killing an innocent person just because of my impulses. For being so stupid. For not allowing a few more seconds. For killing Cho."
Fudge had let him off, saying, "You did what you thought was right. You thought that Cho supported Voldemort. You couldn't have known what she was about to say. Under these circumstances, Harry, I'm not going to send you to Azkaban."
"But," Harry thought, "just because I was stupid enough not to listen, stupid enough to act on impulses, does that make it right? Of course not. I hate Fudge. He and his stupid 'blind spot' for me. I deserved to go to Azkaban." He felt another tear escape his eye. "I killed Cho. I should have gone there.
But he never went there.
Fifth Year
Harry, in a state of numb shock, could only think one thing: "I've killed an innocent person. Cho's dead because of me."
Hating himself for what he'd done, he went to the Minister of Magic's office. "Mr. Fudge," he said, amidst his tears, "Minister Fudge, I've got to tell you something."
Looking up from a pile of papers, Cornelius Fudge said, "Well, it looks like you are a bit distressed right now. What could the problem be?"
The problem is,Harry thought to himself angrily, that I've just killed someone. Fighting to keep his voice steady, Harry told Fudge, "Look, you need to send me to Azkaban for life. I've just used the Death Curse on Cho Chang. A girl in the sixth year." Harry looked right at Fudge's face, which was looking like he couldn't believe what he had just heard.
"Now Harry," said Fudge, "Don't speak nonsense. I would know if you'd just used an Unforgivable Curse. I would receive a message by owl." He had a sort of amused expression on his face, which sickened Harry. A girl's death was not something to look amused at.
Forcing himself to speak to this man, whom he hated with all his heart, Harry said, "I think you've got one, sir. Right there." He pointed to Fudge's right, where a small barn owl was sitting.
As Fudge looked at the message it was holding, his face spread into an expression of shock and horror. "It...it's true," he gasped. "But why, Harry? Why?"
Harry explained it, his voice full of bitterness and self-hatred. When he finished, he said, "Now will you send me to Azkaban?"
But Fudge merely said, "You did what you thought was right. You thought that Cho supported Voldemort. You couldn't have known what she was about to say. Under these circumstances, Harry, I'm not going to send you to Azkaban."
Harry knew he deserved Azkaban. But Fudge refused to send him there. How Harry hated that blind spot Fudge had for him. So instead, Harry left, feeling that not only had he committed a horrible crime, but now he would get off free for it. Just so that he could hate himself even more.
Adulthood
Harry watched as one more petal fell from the rose. He looked at Cho's grave, a small headstone, nothing more. As he knelt before it, he whispered, "I know you can't hear my right now, Cho. But I want to say that I'm sorry. I'm sorry that I did what I did."
He looked at the small headstone again, and added, "I hope you can forgive me. I know it's hard to forgive what I did, but I hope you can. And I hope that I can learn to forgive myself."
He looked at the rose he was holding in his hand, and noticed that all the petals around the outside were gone. They had fell, to reveal the inside of the rose. As he set down the rose, he said, "I hope you can see what's inside me, that I'm not just a cold blooded murderer. Maybe if you can see this rose, and see the inside of it, you will be able to see what is inside me. I hope you can, Cho. Because I love you. I love you with all my heart."