#1: A Shove
There were no true knights in King's Landing that day. Only men with swords who obeyed their self proclaimed King without question. Cruel men with swords, not knights, had slain her father, her Septa, and all the others loyal to her family.
There were no shades of gray in her mind that day. Her father had once told her that the world is not divided between good people and bad people. That everyone has both light and darkness in their hearts. But standing on the steps of the Sept of Baelor that day, as her father was killed with his own sword, Sansa could see no gray through all the red.
Her betrothed had shown no mercy, not one shred of leniency as he had sworn to her he would. Now, he wanted her to see what his broken promise had wrought. He brought her out onto the ledge and forced her to look at the line of head-crowned spikes for however long it pleased him.
They hadn't closed his eyes. In death, her father would stare down at the land he had been loath to rule in life. Her Septa was there, too, and her father's loyal men. People she had loved and learned from, and people who would have died for her. It was then Sansa realized that her father had confessed to treason and swallowed his honor to protect her. She was learning how to play the game, but it was too late.
The edge was so close. It was a mere footstep away, and Joffrey stood within arm's reach of her. There were no handrails on the ledge. It was as if the gods had designed it for this very purpose.
It was a hate filled instinct, not a rational thought that made her do it.
All it took was a shove to end the short lived reign of the boy King.