When Emma saw the dark arms wrapped around Regina, she saw her life since she had arrived in storybrooke flash in front of her. She saw the first time she had met Regina, how she had hated her from the beginning but eventually learned to appreciate the blonde. She saw all the times when she and Regina had saved Henry, all the times they had saved Storybrooke together, she saw the time when she had left, she will never forget the look on Regina's face. Remembering all these memories, she watched as the darkness wrapped her deadly arms around the one that she loved more than anyhting, she saw the fear in Regina's eyes, the fear of dying, the fear of never being the same again, the fear that she would never have time to tell Emma that she was in love with her. Regina caught a glance of Emma's eyes and in a second she saw the fear of losing someone important, she saw the fear of not being able to tell a person how you feel about her. Without thinking, Emma looked a last time into Regina's eyes and put the dagger in the darkness. She knew, at that moment, that she had given her life for the one that she loved.