Massive Killing, Slaughter, Massacre
Noun and する verb
ぎゃくさつ
南京大虐殺が捏造だったというなら、その証拠を説明してください。
If you say the Nanking Massacre is a fabrication, please explain the evidence.
To oppress people and then kill them is an atrocity. It's also called a massive killing because that's what you're actually doing. And you're awful, why would you ever do that?
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.