Yuna felt like her entire world was collapsing. Her only purpose in life was to take revenge for what the New World Government did to Yuno. So hearing that Ren killed the people involved already made it feel like everything she worked so hard for was all in vain. Even though she still wanted to tear down the New World Government, that was only to bring despair to those who caused her brother's death before tormenting them thoroughly.
Yet, none of that mattered anymore. Sure, she could still take down the New World Government, but it wouldn't feel the same. Killing Aira and the Sin Squad was never her true agenda. Killing the people behind her brother's demise was. The people she called the Shadow Society. These were people who did things behind the scenes. They never actively partook in the civil war. They only gave orders here and there.
