I think teenage Murdershy was supposed to be a pony parody of that old horror movie "Carrie" (girl *or in this case pony* wins prom queen, gets pig's blood *or Pony Period* dumped on her, everyone laughs and soon she goes on a psycho rampage *or in the case of Murdershy, killing anyone who comes in her shed*)
That part made me "awww". I felt so much pity for her. But then... I think Rainbow Dash kind of had it coming to her. She basically made fun of Fluttershy for the longest time, abusing her emotionally and physically. Even still, Fluttershy tried to stay true to her element of Kindness. I'm not surprised she snapped a little bit inside. But, that's just my thought on it.