Ringleader
Main Culprit, Root Cause
noun
The origin villain is the villain who's running everything. They're the mastermind behind the villainy going on. They're the ringleader or main culprit.
元凶 isn't just limited to people, so it can often refer to the root cause of a problem too, like 汚染の元凶 (the main cause of pollution). It carries a serious, negative tone and is mostly used in writing about crimes, scandals, or other major issues.
This is a jukugo word that uses the on'yomi readings of the kanji. You've learned these, but since 元 has two on'yomi readings, here's a mnemonic to help you out:
The ringleader of all these problems is Genji (げん). He's the main culprit behind this mess. Remember all that trouble he stirred up by flirting with too many women in "The Tale of Genji"? Everyone admired his beauty, but there's no doubt he was the root cause of a lot of chaos.
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.