What's really sad was that we never got the chance to truly know Yuuki. Other than the fact she was going to die, not too much is known about her. She deserved to live, rather than die within a mere 6-7 episodes. I feel even Reki Kawahara regrets this as well.
I just got into SAO and when I saw Season 2, EP. 18 to 24, I just felt that something was wrong here and these young people ("The Sleeping Knights.") were hiding something about themselves. It was just that "Gut feeling" that you might get at times about something or someone.
Because of this, my heart and soul said, "These kids are all very ill and perhaps even dieing." it was at that point that my heart locked in on little Yuuki and would not let go. Again, my heart and soul said, "This child is far more ill then the rest of this lot, but look at how she is dealing with it." Once it all came out in the open as to just how ill little Yuuki was, (Other then crying my heart out for her the whole time.) I wanted to run to her, wrap my arms around her, draw my sword (I am a swordswoman and archer IRL and I use a rapier myself.) and DARE anyone or anything to keep hurting her! I know that it was a loosing battle, but I would fight it right along side of her as long as I could, right to the bitter end!
This is why I have never worked with ill children to closely. I want to protect them all and my heart and soul take a hit every time one of them dies! My poor heart and soul can not take much more hits like this and it is bad when a cartoon of ANY KIND can do this to me! BUT! This one was and still is a very tough one for me to deal with without crying.