Nightmare Fuel: One of the most terrifying things of the fan fiction is that it's based on something that could happen to anybody. The fact Dash has nightmares over and over causes heavy sleep deprivation that causes her to lose it and murder Pinkie. Holy crap.
Tear Jerker: Rainbow's My God, What Have I Done? moment upon realizing she killed Pinkie Pie. Even worse, it's extremely short-lived, as she goes right off the deep end moments later.
Pinkie as well. Mainly because at the moment she was being killed, all she could think of what did she do to anger Rainbow. Sniff.
What an Idiot!: Instead of telling anypony about the horrible nightmares she's been having and seeking help, Dash keeps it bottled up inside, resulting in Pinkies death and the likely ruination of her own life.