A deliberately caused, carefully engineered mass dieoff, whether of humans or of other animals. A dieoff sounds nicer because it "just happens," whereas a "killoff" requires a murderer or murderers to bring it about.
The authoritarian dictator planned and plotted the killoff over many decades and was now reaping the fruits of victory, worldwide mass genocide or omnicide, more politely termed depopulation.
Guy: “So I went to chick-fil-a and got some nuggies and a large lemonade. It was so good. I kinda forgot to get you some, but you can have some ice that’s left over from my drink if you want.”
Woman giving birth to his child: “Kimloff! You had ONE JOB!!!”