A/N: DEATHLY HALLOWS SPOILERS.
Read at your own risk.
Personally, I think there's another reason behind Molly's rage. Not that she didn't already have enough reason to kill Bellatrix, but...well, we know she lost family members in the first war.
And, just in case it isn't clear, the section in italics takes place 19 years ago.
Arthur heard her scream in anger before he saw her begin to fight. At the moment, he and Percy were still working on bringing down Thicknesse, but Percy managed to get the Minister in a Body-Bind just as Arthur was blocking a curse that the other man had thrown. Father and son shared a brief glance, Arthur's blue eyes asking Percy if he was all right just as Percy's identical gaze told him that yes, he was, so go help Mum.
Others were already trying to rush forward to help Molly by the time Arthur reached the perimeter of the fighting; there were only two battles left. Arthur's blood was pounding, screaming at him to run forward and help his wife, but help was not what she wanted. His mind was still able to think clearly enough to know that he would only distract her by stepping into the fray, and distraction was the last thing she needed right at this moment. It was a moment she had been waiting over nineteen years for.
For the first time in his memory, Arthur was dreading going home. As much as he wanted to wrap his wife and children in his arms after the horrible battle he had witnessed, he knew something even worse was in store for him once he told Molly the news. He could handle her anger without any trouble, but it was much harder to handle seeing her cry.
She was sitting in a rocking chair in the living room, little Percy asleep in her arms. Relief flooded her face when Arthur walked in, and she smiled at him.
"Couldn't get him to sleep in his bed," she whispered. "Wanted to stay up until Daddy came home. This one has been pretty lively, as well," she added, nodding towards her slightly rounded stomach. It would be another week before they discovered she was carrying not one child, but two.
Arthur didn't trust himself to speak. He hated that he had to be the one to tell her this. Why couldn't it have been some Ministry official, or another member of the Order? But no, they couldn't waste valuable resources by sending someone else out to bring her the news. Better that it was Arthur, who needed to be with her anyway.
Her smile faded as he knelt down beside her chair and she noticed the troubled look on his face. "Arthur, what is it?" she asked, her voice slightly louder in alarm, and the sleeping child stirred in her arms.
"Molly..." he began, pausing to swallow back a lump in his throat. The prospect of speaking the words aloud made him realize that they really were gone, and it was a hard pill to take. He had loved them, too. "Molly-girl, Gideon and Fabian...they didn't..." A tear slid down his cheek, escaped from the fortress where he was trying to hold its companions. "I'm so sorry, Molly."
"No." It wasn't a word of protest, or even disbelief. It was a firm statement, as if he had just asked her to come along with him to a Muggle football game and she was denying him. Without another word, she heaved herself out of the rocking chair, Percy in tow, and started up the stairs.
Not knowing what else to do, Arthur followed her. In spite of being pregnant and holding a sleeping child in her arms, and in spite of the fact that her legs were much shorter than his, he couldn't seem to catch up with her on the stairs. She went straight into Percy's room, laid the little boy down in the crib that he would be abandoning in a few short months, and left again, shutting the door behind her. She tried to squeeze past Arthur on the narrow staircase, but he stopped her, gripping her plump shoulders firmly in his hands.
"Let me go," she muttered, not meeting his eyes.
"Molly, I saw them. I saw it happened," Arthur tried again. "They...they went down very bravely, Molly. If it weren't for them, five Death Eaters that were taken into custody tonight would never have been captured."
"Let me GO, Arthur," she repeated, nearly shouting. He had a feeling she would have been screaming at him if it weren't for the three sleeping boys nearby.
He obliged, and she started down the stairs. She made it to the landing before he managed to fish what he was looking for out of the pocket of his robes. He was afraid she might need some sort of proof like this.
"I kept Fabian's watch," he called after her quietly, holding the timepiece up for her to see. She froze on the spot, gripping the railing so tightly that her knuckles seemed to glow white in the darkness of the hall. Slowly, she turned to look, her eyes fixed on the watch in his hand. "Gideon's wasn't...there wasn't enough..." He stopped himself, unable to finish the thought - there wasn't enough of Gideon's arm left to even make out his hand, much less save a watch.
Time seemed to stop for a moment; if it weren't for the continued ticking of the timepiece in his hand, Arthur would have thought that it had. Then, with a strangled sob, Molly crumpled down to the floor of the landing, her hand still clutched onto the rail.
Arthur was beside her in an instant, his arms wrapped around her tightly, rocking back and forth. He didn't know how long she cried, how long they sat there together, but her sobs finally subsided. They sat together in silence for a long while, clinging to one another, before Molly finally asked what he had been sure she was going to.
"Do you know who did it?"
He hesitated. Death Eaters tried to keep their identities a secret, but there were a few who were known. There were even a few who were reckless enough to barely conceal themselves. "They wear hoods, Molly--" he began, but she cut him short; she could already tell that he knew.
"Who was it, Arthur?"
He took a deep breath, looking into her eyes that were already so full of hatred for the unknown person who had killed her brothers. She had a right to know.
Arthur stood on the brink of the battle, still ready to step in at a moment's notice if he thought his wife needed him, although he knew she probably wouldn't. Molly had been considering a career in the Department of Magical Accidents and Catastrophes before they had eloped together (being a mother to seven rambunctious magical children had turned out to be nearly the same job description). She was a skilled, powerful witch, and hatred for the woman she was fighting had been boiling up inside her for years. With the loss of Fred so recently added to her fire, he knew she could handle herself. Revenge was a powerful emotion.
"You - will - never - touch - our - children - again!" she shrieked, and suddenly Arthur realized that revenge wasn't all that was fueling Molly. He knew, although others may not have recognized it, that she was talking about more than the loss of Fred at the hands of Death Eater's, more than Bellatrix's attack on Ginny, more than even the murders of her brothers.
She was talking about Harry Potter. The Boy Who Lived, the boy who had so recently died trying to save them all, the boy Molly had never hesitated about bringing into their family in spite of the danger he brought with him. The boy she had considered a son just as much as any of the other six boys she had given birth to.
Revenge may have been a powerful emotion, but a mother's love topped it by far.
The curse hit Bellatrix square in the chest, and she toppled. Relief flooded through Arthur, but it was immediately replaced with panic; the Dark Lord himself was raising his wand against Molly. He opened his mouth to yell out a shield, wand already raised, knowing full well that he wouldn't be able to get the shield between them from the angle he was at -- and suddenly, a shield appeared out of nowhere, and Molly was thrown backward into the watching crowd from the force of it. He rushed foward to help her to her feet, only to jerk his head up back to the center as she shrieked with joy.
The hall immediately fell silent. Arthur held his wife so tightly against him that he thought he might be in danger of crushing her, but she didn't complain. Hearts pounding against one another, they watched as the confrontation unfolded across an eternity of waiting...and then it was over just as abruptly as it had begun. Molly was crying again, this time with joy, as they crowded around Harry with the rest of their children, and she was once again the image of the plump, doting Weasley mother.
Yet somehow, Arthur knew none of them who witnessed her defeat of Bellatrix would ever look at her quite the same way again, and in a way, he was glad for it. They finally recognized her as the powerful witch he had always known her to be.
Love was far more powerful than anything else, even power itself, and Molly Prewett Weasley took no prisoners when her family was threatened.