Approximate kill count rankings, not counting shit which got reverted like restarting the universe or whatever*:
1) Starlight Glimmer
>single handedly caused demographic change with her serial killing
2) Twilight Sparkle
>dropped the sun on manehattan
>state executioner
3) Rainbow Dash
>unleashed the atomic rainboom on cloudsdale
4) ??? (Soarin? or one of apple split's children or flawless? I swear this happened but I don't remember when)
>destroyed manehattan the second time
5) Princess Cadence
>killed everyone in the changeling kingdom (changeling lives only count for 3/5ths of a pony, though)
6) Trixie Lulamoon
>killed half of ponyville by shitting in a cloud
7) Care Package
>kills a family at least once a week
*Trixie, Starlight, Care Package, and Cadence/Chrysalis have all killed everyone on the planet, but they came back so it doesn't really count.
What we can learn from this that Feminism is the leading cause of death in Equestria, followed by autism and bitchyness.
I do think it's funny how Flawless being pure evil is literally just a product of being a mix of her parents because they're literally like top 5 worst peeople on the planet even including Celestia