>Killcounts
Twilight:
>Gored a pony with her horn while exerting Alicorn supremacy
>Dropped the Sun on Manehatten
>Gored Flawless in the mind realm
>Honorable mention: Trusted by Celestia to kill on several occasions, all Gay Stallion related. Never succeeded though.
Rarity:
>Accidentally finished off Applejack after she was gored by Flawless
>Finishing blow against Flawless, shared with Sunset Shimmer
>Responsible for a stack of bodies under Sweet Apple Acres
>Caved in a skull of a carriage driver
>Honorable mention: Shot Granny Smith in the head, intended to be lethal.
Rainbow Dash:
>Atomic Rainbomb on Cloudsdale
>Accidentally shot Fluttershy in the head.
The Other 3:
>Surprisingly nothing, other than Pinkie Pie sending Applejack to the faceless mare.

I'm not saying it's all unicorns, but it seems to be mostly unicorns to be wary of.