Before starting lets set the scene; Book: Order of the Phoenix (movie); Scene: Department of Mysteries right after the battle with the Death Eaters and the Order but right before the battle between Dumbledore and Voldemort.
What if things with Harry and Bellatrix had went just a bit differently?
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Standard Disclaimer applies as well as note; some of the text comes directly from the movie.
"No, no, Sirius!" he screamed into the cavernous darkness. He looked around to find him amd saw her instead. An evil smile crossed her face as their eyes met. She slipped away and he fought until he was able to wrench from Lupin's grip. He ran down the inclinement, slipping into the same pathway she had used to escape. He could hear her voice as it echoed through the hall.
"I killed Sirius Black," she sang as she ran along. She looked back and saw Harry coming behind her. "You coming to get me?"
"Crucio," he called after her as he pointed his wand in her direction.
She stopped laughing as she met the floor to her own disgust. He was not strong enough to knock her down but her own desire to taunt him caused her to loose focus and fall. She prepared to get up and turned to look at him. She smiled at Harry, a horrible, despicable smile.
'You've got to mean it. She killed him. She deserves it. You know the spell, Harry.' It was a faint voice in his head but he heard it repeat and repeat. Though he could not pinpoint the voice it was familiar yet daunting. He wanted to block it out but the his pain was unbearable, losing Sirius hurt so much and he didn't want to deal with the pain, he just wanted her to suffer for what she had done.
"Crucio!" he repeated as he caught up to her. He stopped and looked at her head on. She smiled as she saw a flicker of color in his eyes.
'She killed him. He was the last, now you have no one.'
"Crucio!" he screamed out in pain. That time she writhed for him. He repeated, "Crucio!" He watched as she clutched her stomach and retreated into a fetal position. "Crucio!"
"Master, please!" she cried out expecting mercy from he who she has served with her life.
Though Harry looked directly at her, she was not what he saw. The vision of Sirius' face as he slipped into the veil tormented his mind. Watching his body just disappear into veil, gone forever. He heard the words in his mind over and over again, 'She killed him…You've got to mean it.'
"Crucio!" he said again and again watching her writhe in agony, refusing to give him the satisfaction of crying out in pain.
'End it Harry, make her pay.'
"Avada…"
"Harry stop!"
It was the voice and hearing his name that that brought Harry's head up. He twisted toward the direction the voice came from.
'She killed him. He was the last, now you have no one. End it Harry, make her pay.'
"I know you're hurt but you cannot do this."
"She killed Sirius."
"And you will kill her, will it bring Sirius back?"
'She killed him. He was the last, now you have no one. End it Harry, make her pay.'
"She killed Sirius!" he screamed at the beloved headmaster.
"Yes, I've heard you."
He heard her trying to scurry away and turned back to face her. He was angry at Dumbledore for coming now. His head was hurting and a dull pain was trying to set in. He rolled his head around then from side to side. Harry took a brief look at Dumbledore then back at her. He had had to do this, she had to pay; first Neville's parents now Sirius. She may not have paid for what she did to the Longbottoms but he would make her pay for Sirius.
'She killed him. He was the last, now you have no one. End it Harry, make her pay.'
"Crucio!" He meant it, with every fiber of his being, he meant it. He wanted her to feel his pain, all 15 years of it. He turned his frustrations, aggravation and insecurities into anger and pain. It coursed through his whole body as he held his wand with ultimate determination.
"Crucio!" Harry and Dumbledore watched her body twitch at a ridiculous speed before she tucked herself back into the fetal position, her head still moving from side to side with a slight tick. She looked deranged as her body moved involuntarily. Her body moved in silence and her eyes were crazed but she didn't cry out. Watching her made the dullness creep more into his mind but also fueled his anger. She would cry out in pain. He would make her voice his pain.
"Crucio!"
"What of Neville? Should I get him and leave the two of you to torture her together until she dies?"
'She killed him. He was the last, now you have no one. End it Harry, make her pay.'
"She killed and Sirius!"
"Who, is at least at peace. Frank and Alice Longbottom are stuck in their own minds barely recognizing those around them, yet you feel you are more entitled. I believe I should…" They both turned when they heard a noise to find Bellatrix Lestrange gone.
"No! See what you've done!" Harry screamed out.
"Yes, did you? Harry, listen to my voice. Listen to me Harry. This is not what Sirius would want. Harry please."
Harry felt what could be described as a fog lift from his mind. The voice stopped and instead of the mind-numbing pain he was hit with an awful wave of grief. The dullness had taken over his mind. No voices just the reality of what was happening, of what had happened. Sirius, Oh God no, not Sirius too. But the reality for Harry was that Sirius too had gone and left him here alone; with no one. He fell to the floor on his knees; tears fell from his eyes like rain.
"Sirius," he whispered as he looked at the ground and covered his face with his hands.
"Harry, I'm so sorry. I know that you and he were developing a very strong relationship. If there was anything I could do now for you I would but you are just going to have to grieve yet another loss." He was going to continue until he heard a sound. He turned to find none other than Voldemort behind him.
"It was a mistake for you to come here tonight, Tom. The Aurors are on their way."
"By which time, I will be gone and you will be dead."