It hit Draco all of a sudden, what exactly Theo being her tether meant. It meant that as long as Theo was around she would be anchored to being good Hermione. But that girl couldn't do what needed to be done, whatever her mission was, she needed to be fully able to give into her dark side to do it. The side that rarely showed when Theo was around, the side that took over when he was away. If Theo died, the good part of Hermione would be lost forever, and if the good Hermione was lost.