Alternate Universe - HP Edition - retelling of {SPOILERS} Penny's death in Volume 8 Chapter 14.
Jaune has to mean it - with everything inside his body he has to want it.
Penny Polendina screamed and everyone's eyes slowly fell to the Grimm claw embedded in her chest to draw on her power.
Time seemed to slow to a stop as Weiss Schnee cast her time dilation spell with her pureblood runes, lurching forward as she swung Myrtanester to send a slicing spell on Cinder Fall's demonic limb. Cinder snapped back her limb just a fraction of a second before it was shredded into two, but had no time to return to the mortally wounded as the last standing of team RWBY went in head first with her teammate's Gambol Shroud, throwing everything she had left at the woman that had just killed her best friends.
'Penny!'
Jaune Arc ran to her side, hands aglow with his magic ready to replenish her reserves.
But Penny had other plans - holding his wrist and shaking her head weakly.
'There's no time Jaune.'
'I can fix this Penny, please!' The tears in his eyes couldn't fall. 'Let me save you!'
It would be just like Beacon again, she had said back then. And like Beacon it was, with Penny taking the fall, again.
He couldn't let Beacon happen. Not again.
'We can't let Cinder have this power as well.' Penny smiled at him softly, vaguely aware of how her words of so many days ago, said with so much hope, was ironically back to hit her in full force. 'Let me choose.' She lifted her finger to his lips and stroked the corners faintly. 'Let me have this one chance to make a choice, Jaune. To make this right.'
One tear escaped the corner of his eye and fell on to her thumb, but he didn't even know it as he stared at his friend helplessly.
'Penny please...'
But she only smiled at him, her hand falling to brush the helm of his sword. She gave a tired nod of encouragement.
He pulled out the weapon and stared at his reflection in the blade. But he couldn't look at himself, knowing he was even thinking of making this choice. Ruby Rose would never have made this choice.
Pyrrha Nikos would have though.
He turned away from his sword and cast it aside, his eyes on Penny again.
'You have to mean it,' Ozpin had taught them about the greatest unforgivable curse a lifetime ago, when he was a man and not a young farmhand, when they were students and not foot soldiers. Pyrrha was alive, and Yang had two arms, and Penny was a girl whose body protected her from the mortality of being flesh and blood. 'You have to mean it - with everything in your body you have to want it.'
'So you have to hate someone to cast it?' Cinder had asked then when she was just a student of haven. A murderer and a psychopath among them, hiding in plain sight. The grin on her face had been unnerving even then, a glimmer of her true sadism being suppressed.
'...you can hate them enough to truly want them dead...' Ozpin raised a brow then, no more than wary of teen angst. Oh, the irony, that he had not been warier then. 'But hate might not always make the desire for it to happen true. Love is just as strong, mercy, desperation for a greater goal. The killing curse has no meaning without intent.'
Cinder could not have this power. Not the woman that had murdered Pyrrha for it. That had tried to do the same with Penny and put him in this place.
Not after RWB had died.
Not when Atlas was falling.
This was war and Penny would not die to further the plans of his enemy. Not in this life or any other.
Penny had this one chance to go on her own terms. Just like Pyrrha had. She had done the opposite of what he was about to do. She had chosen to give where he had to chose to take away.
'Jaune,' Penny looked him in the eye, with a gentle smile. 'It's okay.' Her hand fell to his and squeezed gently, 'I thank you.' He could only swallow thickly.
Penny was choosing to give too.
He opened his mouth to speak, the hands once covered in healing magic about to take their first life.
The life of a friend.
And as he said it, he really meant it with everything he had left - whatever that was. So it was just as well that he would cast an unforgivable rather than use a sword.
Because he would never forgive himself.
'Avada Kavada.'
...
Jaune's scream was louder than Penny's had been.
But when Cinder saw what he had done, she screamed the loudest of them all.
Where are my priorities? I've never written a one-shot before, sue me. It was something that came to me after going through an HP subreddit on how unforgivables are misunderstood.