1. What does it mean?
빌런 comes from English 'villain.' Besides fictional antagonists, Korean conversation uses forms such as 소음 빌런 for someone repeatedly causing a nuisance.
Also written or said as billeon
2. The nuance
The joke can slide into insulting or shaming a real person. Describing the specific behavior is more informative than turning someone into a villain, especially without knowing the circumstances.
3. Examples
“도서관 소음 빌런이라는 말보다 조용히 해 달라고 직접 부탁했어.”
Instead of calling them a library-noise villain, I directly asked them to be quieter.
“이 영화는 빌런의 동기가 설득력 있어서 더 흥미로워.”
This movie is more interesting because the villain has a convincing motive.
Sources
Sinjoeo Korean Slang Dictionary
References support the meaning and usage. Example sentences are written for NoCapWiki.