I always had wanted to try and portray the reasons behind Mami doing this, based around it being that she cared more for Kyouko than she let on in that timeline.
I died the instant the arrow shattered my Soul Gem. Moments before, I'd held the pistol, my hands shaky as I aimed at the person I cared for most. I couldn't bring myself to pull the trigger easily. Tears stung my eyes, and blurred my vision. Even though it was hard to see, I knew exactly where to aim. I shook with fear, knowing very well what it was capable of doing to a witch or familiar. Then again, I knew what a Puella Magi was capable of becoming as well. We all knew, but it was much too late.
I only wanted to prevent the others from becoming what Sayaka had. This was the truth that Kyubey had not told us because we had not asked, or perhaps even considered it as a possibility. Homura…had told us once, but we hadn't truly believed her at the time. Sayaka had fervently been against it, and in the end, Homura's words came true. Puella Magi become witches when they lose all hope and their Soul Gem darkens completely.
Kyoko had been the one no one trusted at first. She was just some girl, another Puella Magi, who wandered the streets. I happened to meet her on my way home one evening. Something about her made me talk to her, which turned into an invitation for tea and cake. I couldn't explain my reason, but it was simply something I wanted to do. She readily accepted, though I think it was more for the offer of food, rather than companionship. Still, she at least made small talk while we ate. When she left that first night, I didn't think I'd ever see her again.
It surprised me when she continually met me after school. On the days that I walked home with Madoka, Sayaka, and Homura, she stayed hidden. It took some time before she warmed up to those three. After we'd parted ways, Kyoko would soon reveal herself, and we'd head to my apartment for tea and cake. It became routine quickly. The one night I made dango instead of cake. Kyoko and I talked long into the night, until she eventually fell asleep at the table, her arms creating a pillow for her head. I covered her with a blanket then, noting how serene she looked at that moment.
These were the memories that had plagued my mind as I had taken my aim. No one had noticed as I loosed my ribbon from around my neck. They were all too upset over destroying Sayaka. I had sent the ribbon toward Homura, binding her arms together so that she could not stop time. Without another thought, I had pulled the trigger, and watched Kyoko's eyes dull. The twinkling of the shattered Soul Gem echoed, and she was gone. In my despair, I spoke my reason. We had no choice. We needed to die in order to save ourselves from the alternative. And just like that, my Soul Gem was shattered.