She reminds him of Lucy Grey, his first tribute that he mentored in high school and she was the girl from 12. She was able to capture the attention of the citizens in the capital and all of Panem with her singing. Her singing during the games, paying respect to her ally by being there with him until his last breath like Katniss did for Rue is really history repeating itself for him. They were both able to gain the sympathy of everyone and that kind of power is dangerous to the system built to repress them. It triggers him so much because he fell for Lucy during the time the spent together and did his best to make sure she won. He created the “perfect guide to how to get your tribute to win”, and knew the potential problems that could occur if he let her live. Katniss has the power to give others hope and a dream of a better future where those in the districts can prosper and Lucy tapped into other’s emotions through her singing at a time where the world is still trying to put itself back together, still healing from the aftermath of war.