AN: I've always had this theory that South was the one that killed North, though, unwillingly.
It just seems like something Sigma would do, take over her like that and play with her jealousy enough to want North gone forever.
This is old, and I know I should probably touch it up or add to it or something, but that requires effort and energy, things I seem to be lacking a lot at the moment.

It was a simple shiver that ran up South's spine. Her eyes remained locked out in the distance, just watching while North did whatever it was he was up to in the back. He never did tell her what he was up to, not since they left, he only ever told Theta; hell, it was probably Theta giving him the ideas.
"North?" It was odd, that after all this time, even though they were related, they still went by their code names. She couldn't stand the not knowing, she hated that she didn't know. Wasn't being his twin enough to trust her?

The fact that it hurt was odd, shouldn't she have locked this feeling away years ago and become used to the ugly head it bore? He needs to know how it feels. Her mind seemed to whisper the ideas to her, manifest the images of what she would do.
"I'll just be two minutes." The call came from the room she was stationed outside. Noises could be heard within as he continued to search for something. He must know the pain. It was without thought that her sheening blade was drawn from her belt.

His back was turned, it could have been so easy to slip the silver icicle into the tender skin on his neck and be done with it, but he at least deserved to know, to see who it was that had ended his life. With steady hands she lifted them up and slowly removed her helmet. He needs to look into your eyes. This voice, it seemed to be commanding her thoughts, her actions; just carefully hitting the right switches. It fell to the floor as her arm dropped to her side; it was an announcement to her brother that she was there, that he was there. All he did was throw a glance over his shoulder, not even taking in the fact that she bore a blade, ready to end him.
"I told you, I'll just be two…" He paused and finally turned to face them. "South, are you okay?"

No answer was given, she simply lifted the knife to delicately finger it; he held the tip of the blade against her finger and began to twirl it. That look in her eyes was all he needed to know; no one knew his sister better than he did, and when she wanted to kill someone, North knew. It had hurt to always be second best, he had to know that, just this once, she would come out on top; and this time, she would stay there.
"South, you don't want to do this." What did he know?

He was her brother, he knew everything. Her twirling faltered once.
She shouldn't listen to him; he would just hinder her success. It didn't mean he had to hurt. Her arm lowered slightly.
He would chase her down; he could never let her go. Her blade returned to its sheath, leaving an audible sigh of relief from North.

There was a brief moment where the two siblings merely stared at each other, South blinked once before visibly shuddering; her body moved in harsh jerkiness. Something within her changed, it was no longer North's dear sibling looking at him, it was someone, or something else, South had never had such a look of malice or anger saved for him. It took less than a second for him to whip her pistol out and take aim, hands visibly shaking. It was her hands shaking, but there was nothing anyone could do now. His cruel smile curled her lips as in one, fluid motion they brought the pistol to his one kink in his armour and North's body fell to the floor. South could retain enough of herself to understand that the loud thud was the sound of her brother, and only real friend falling to the floor, never again to share any of her burdens. Her body shuddered once more.

Now, she was alone.