The prophecy Susie read is that she has to kill Kris
It's the only thing that really truly makes sense
She wouldn't have been as mad in the same way if she was the one that dies. Particularly, if Kris or Noelle had to kill her, she'd scoff.
But HER being made to kill her new best friend, to be commanded to by fate, nothing could possibly piss her off more. To finally find this family and have the world tell you it will be taken away by your own hands. That would make Susie angry enough.
It's not Ralsei getting killed in the prophecy because he wouldn't have been so apologetic or ashamed if he was the victim.