I feel like the ending was just the end of an unhealthy cycle. Sae wanted somebody to make her the center of attention, and Riko wanted somebody to see her from a different perspective from everybody else. When Sae realized she was the center of Riko's attention, she realized why she hung out with her without realizing how much she hated her until the end of CH. 1. So, she no longer hated Riko, but she also realized she'd be just like everybody else to Riko. So, Sae forcibly took Riko's picture to express to her the new perspective/image of Riko she had. Then Riko was unable to see Sae as anybody special, anymore. It's a bad end, because both characters are thrown into worse situations than before. Riko no longer has anybody special, and Sae is COMPLETELY alone as far as we the readers know. The conflict of the story only got worse, but maybe that was the intention.