All Homura wanted was to save Madoka. It seemed simple at first. Just go back and help her defeat Walpurgisnacht. It soon became clear that becoming a magical girl would spell Madoka's doom. But that just meant that Homura would just have to go back a little earlier and keep Madoka from making a contract. Her first few attempts failed. It was no big deal. She could just go back and try again. Only, each time Kyubey seemed a little more insistent. At first, Madoka was just an ordinary magical girl, but each loop Kyubey made it more clear that Madoka was somehow special. Eventually, Homura found out why. When Madoka inevitably agreed to the contract, Homura would wait until they fought the Walpurgisnacht to go back. Each time, Madoka got a little bit stronger.

This wasn't a problem. Sure Kyubey would get more insistent, but Madoka would get more and more practice. No matter how important Madoka became, Kyubey could only try as hard as he could, but Homura would have all the time in the world to learn and grow, or even to just get lucky. But Madoka's growth was exponential. Soon, it meant more than Kyubey trying harder to make a contract. Madoka was getting increasingly less likely to die a magical girl, and increasingly more dangerous as a potential witch. Homura redoubled her efforts. She began to wonder if she should never have tried to save Madoka. But it was too late for that. Time travel can't undo earlier timelines. She had to save Madoka.

Homura watched Madoka make a contract once more. Madoka killed the Walpurgisnacht with a single arrow. Then she became a witch. Kyubey told Homura that it wouldn't be long before she destroys the world.

Whenever Madoka made a contract, it didn't stop Homura. She'd just go back and try again. Running out of magic wasn't an issue. Homura had hundreds of grief seeds collected. If she ever started running low, she could just be a little more conservative with magic. But there were other dangers. Homura could be killed. She mitigate the danger with more liberal use of magic, but not if she wanted to keep her store of grief seeds. If Madoka's power kept increasing, Homura might not have time to go back before Madoka killed her as a witch. If it kept increasing long enough, Madoka would kill her as collateral damage.

Madoka meant the world to Homura. But that saying was not meant to be taken literally. The world was more important than Madoka. The world was more important than Homura's feelings. Homura finally decided that it really would have been better had she left Madoka for dead from the beginning. Once she understood that, she realized she'd been willfully ignorant. It wasn't too late to undo her decision. She'd never thought of how before, because it was unthinkable.

Madoka slept peacefully in her bed. Homura stroked Madoka's hair with her left hand. She brought the muzzle of a gun to her Soul Stone. And for all but two people, the world did not end.