I feel this was/is an illusion to trick Yuki into hating himself. Yeah,a little bit of a dark turn...Maybe the mirror on the right is what is ahead in his future unless he changes! I don't know.
@Grant Alexander Scott Hehe, it's interpreted in any which way fits. ♥ Also Yuki's a male Uxie but he's also gender-fluid so it doesn't matter if you call him a 'her'. 8D
The way I see it, Yuki is seeing not only her present self, but the innocence of her past self as well (left mirror). The right mirror tells me that either it shattered due to what was seen in it, or Yuki herself shattered it after seeing something of herself in the reflection.
To me, this is a powerfully emotional piece of work, and I thoroughly enjoy it.