No cosmics or aliens allowed here (bye-bye Anti-Monitor, Darkseid, Parallax, Death, Lucifer Morningstar, etc).
Direct and indirect. Across all main comic book continuities (golden age, pre-crisis, pre-flashpoint, New52, Rebirth)
Figure the automatic contenders are
Deathstroke
Deadshot
The Joker
Lex Luthor
Ra's al Ghul
Vandal Savage
Others?
Whose on-panel civilian body count is highest over the years?