Nightmare Moon is Envy, because… duh.
 
Discord is Pride, because he looks down upon other ponies, sees everything as a “game” and his pride leads to his downfall.
 
Chrysalis is Lust, because she thrives on the power of love, and quite literally sucks Shining Armor dry. Ugh.
 
Sombra is Greed, because he craves “CRYSSSSTALLLSSSS” and power, no matter how he achieves his goal, be it through slave labour and harming others.
 
Tirek is Gluttony, because he seeks to absorb all magical energy, and because he, quite literally, eats it.
 
Sunset Shimmer isn’t anything, because she sucks.