Ringleader, Main Culprit
noun
The origin villain is the villain who's running everything. They're the person responsible for the villainy going on. They are the ringleader or main culprit.
げんきょう, がんきょう
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
人類の半分を一掃した凶悪なウィルスの元凶は、ウォッカのつまった西瓜だった。
The source of the terrible virus that wiped out half of mankind was a vodka-filled watermelon.