Implied Blackcest. Written for 15minuteficlets; word: Competition.
That's all it really came down to, Sirius is absolutely bleeding sure, all those scathing comments were never because of his actual traitorous actions, it was only competition. Bellatrix sought out competition in any form she could, everyone knew that—as confrontational as she was, there was no doubt that she actually did live on spite alone.
It wasn't as though she ate. Sirius watched her pick at her food at family dinners when he was actually invited, watched her scant figure and knew she wasn't eating. It was only verified when she was going to get married to Rodolphus, and Andromeda frowned as she told him rumor said the dress was only to be tailored because Bella didn't care to eat. There were, after all, bigger things to focus on than herself.
Later, she gained weight, likely because Aunt Druella made certain she wouldn't do otherwise.
She had to be the prettiest. The most well-read, despite that she barely had the patience to read the whole of books if she could just get the idea without reading all those tedious pages. The smartest, the best duelist, the fastest draw of the wand, and he challenged her on every step.
She had to win, most of all, over him.
When they were young she smacked him around, boxed his ears. She did have one hell of an arm for a thin little girl, though. He tried to hit her but found her hands pinning him down. Her waifish form knelt over him, easily holding him down when all he should have had to do was push her away.
He struggled, but like some sort of black widow—oh, what an apt name—she held tight.
"Bitch," he'd growled, and leaning over him, she'd said a comment he'd never quite grasped:
"You know you love me for it, Sirius."
He'd lost every battle since.