Vasily Blokhin
Executed 7,000 people. Not simply ON his orders, no: I mean he executed them PERSONALLY.
On one day he did 300 one by one, all of them officers.
This was one of the key executioners in the Katyn Massacre, of the Polish officer corps.
For those who have not seen it, its on Youtube. I recommend watching it. https://www.youtube.com/watch?v=Q2ZYdiEE20Y
I believe , in the end he blew his own brains out.