You have to mean an Unforgivable Curse for it to work, but what if someone else means her spell more? A missing POV story.
Stalemate
"Avada Kedavra," Pettigrew snarled as he cast his curse at Luna. She blinked, and went back to tending Mr. Ollivander's injuries. The green flare of the curse had faded before it reached her. The sound of the curse was missing.
Bella waited outside the cell's door. "Well, when are you going to bring out the body?" she coldly asked.
Peter swallowed and shifted his weight from one foot to the other. He didn't dare aim in the direction of the trembling wandmaker, knowing that he was still needed. However, the failure of the capture of Harry Potter at the Lovegood residence demanded that the girl be killed as punishment.
"Avada Kedavra." It was the same as before, only she didn't even blink this time. His confidence was shot. You have to mean the spell, he remembered from his school days a lifetime ago. Well, he obviously didn't mean it, although he was sure Bella would if she took a mind to kill him.
"Uh, Mrs. Lestrange," he whined. "I've got a cold, and my spell isn't working. Could you…"
She was there in a flash, angry at his incompetence and eager for a kill.
"Avada Kedavra."
The noisy green flash hit Luna, and then spread out, like fermented moss juice dashed on a wall, and then faded away. Luna evenly gazed back at them, while the wandmaker fainted and fell into her arms. She gently placed him on the floor and arranged to make him more comfortable.
Bella tried again and again. Enraged, she screamed at Luna. "Die, damn you! Why don't you die?"
Luna ignored the rude question as she set to work tidying up the cell by moving the bucket to another corner and picking the straw off of the old man's shirt.
Bella steeled herself. "I order you to die!"
Luna looked at her, "No, I think not," she replied.
"I mean for you to die!"
"No," Luna said. "It's just not my time. And I don't mean to go before then. I'm in no hurry." She went back to her self-appointed tasks, humming a child's tune.
Bella stomped over, and banged into a non-verbal shield charm. Luna continued to ignore her. Peter stared in amazement.
No one should survive that curse. And yet Harry Potter did, and now this girl. If word got around that the curse could be rendered ineffective….
Bella glared at Luna, and realized that the human rat was still watching her. She knew that, as Voldemort's literal right-hand man, she couldn't cast an Obliviate at him, without the Dark Lord knowing it, and that he would betray her in the blink of an eye if she threatened him.
There was nothing left to do, but to leave well enough alone, and hope that if the rat blackmailed her, that she could deal with him then. Bella Lestrange swept out of the cell. Peter, after a pause, scampered after her, terrified of the girl.
And in the darkness, Luna continued to hum.
