This wasn't how it was supposed to be.
He wasn't supposed to have his sword through her back, coming out of the other side of her chest, and her lifeless eyes staring at him asking him 'why?'
He knew that Aizen was a bastard. He knew that, and yet part of him couldn't believe he would take it this far. Why him? Was it because he was the first to notice something was going on between him and Ichimaru? Was it because he was so young and considered a prodigy, more than any who had come before him at the Spirit Academy? Or was it simply because she devoted herself to him so much, that he wanted to rub it in Hitsugaya's face? No, he had never let Momo go, despite how it may have looked on the outside. He tried to lock away his feelings for her, because he felt he didn't deserve her and he never thought he stood a chance to begin with, but that was far from meaning he didn't care.
But what did she see in him now, now that he was killing her? Now that his zanpakuto was literally inches from her heart? He briefly wondered if it was still beating. Kira was healing her, but with her injuries earlier and now this, would she survive?
A wave of rage made his blood boil. Aizen would do this to her, after everything he already put her through? No, he realized, it wasn't to hurt her... it was to hurt him. Toushiro wanted to crush Momo's body to his and scream until his lungs bled. He wanted to slowly slice Aizen into pieces so he could feel every bit of pain he had caused Momo. He wanted everything to die so that she could live, even if it meant he would never see her again; for now that his sword had pierced her flesh so, he knew he didn't deserve one glance from her, one second of her breath or energy focused on him.
He wished time would freeze, so that he could think about what to do. He didn't have that luxury, and the only feeling he had was to cut down that bastard, and stab him repeatedly until his corpse was nothing but dust. Even then he knew he would rage, but it didn't matter—he had to do it for her, avenge her, and make sure any and all who dared to cause pain upon the most beautiful light he had ever known would suffer for such a sacrilegious transgression.
And with a thunderous roar that couldn't come close to the one in his heart and soul, he flew forward into oblivion.