For more info on the anime just click the link below.  
Twilight Sparkle: Killing with knowledge or Killing with magic  
Applejack: Killing honestly or Killing by bucking  
Rarity: Killing fashionably or Killing beautifully  
Rainbow Dash: Killing while looking cool  
Fluttershy: Killing shyfully or Killing with critters  
Pinkie Pie: Killing Psychotically or Killing with a smile  
Flam, the elder Flam Flim Brother: Killing for bits or Killing by scheming  
Flim, Younger Flam Flim Brother: Also killing for bits or Killing with deceptions  
Gilda: Killing while being awesome or Killing while flying  
Nightmare Moon: Killing from the shadows or Killing pridefully  
Sombra: Killing with fear  
Garble: Killing with force  
Spike: Killing with fire  
Ember: Killing with supremacy  
Grover the Diamond Dog: Killing by digging  
Iron Will: Killing with strength or Killing in a rage  
Celestia: Killing with rules or Killing with light  
Zecora: Killing Mysteriously or Killing with skills  
Prince Blueblood: Killing while being perfect (self proclaimed)  
Queen Novo: Killing by drowning  
And as for the roles of the Zodiac Criminals from the sequel can mostly be MLP villains from all generations since they’re criminals.