These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
To clarify: Not long before the events of the story, they had been dating Mikuo and Len. But the guys became disillusioned with their rotten behavior quickly, and soon broke off the relationships. Unfortunately, Zatsune and Arakawa absolutely refused to believe that they were at fault and instead convinced themselves that Miku and Rin were somehow responsible for the end of the relationship and that they would have to pay dearly. So right in the middle of the concert and in front of over 16,000 fans (more than 5,000 of whom were children), they made their move by brutally attacking Miku and Rin on stage, leaving them both badly injured.
Moral Event Horizon: If Zatsune and Arakawa didn't cross this with their uncalled for attack on Miku and Rin, or when they activated an electric barrier around the stage, they definitely crossed this line when they trapped the audience, of which nearly a third of whom are children, in a wall of fire.
Zatsune crosses the line even further when she escapes from prison and decides to eliminate Miku, once and for all.
It's especially bad for Rin, for she ended up passing out on the way to the hospital, then she learned she would have to undergo surgery and was reduced to a nervous crying wreck when she found out (She had never had surgery before). If you didn't feel bad for the poor girl, there's something wrong with you.
Miku gets another major moment when she realizes that a homicidal Zatsune has broken out of prison and is rapidly making her way to the hospital to kill her.
All those poor people in the audience during the concert, especially the 864 people that were unlucky enough to suffer injury. They had merely come to see the Vocaloids perform live in the park, but soon they ended up being forced to fight for their own lives.