[6] Yeon's first feature-length animation was The King of Pigs (2011), about a man who kills his wife after his business goes bankrupt, and seeks out his long-lost friend, a ghostwriter, 15 years after both had been severely bullied as adolescents in middle school.

Inspired by the works of Satoshi Kon and Minoru Furuya, Yeon said the incidents in the film were drawn from his own life, and he cried while writing the screenplay.

[7][8] The low-budget (US$150,000) film drew widespread critical acclaim for its raw portrayal of bullying, violence and systemic poverty (and the lifelong effects of such oppression), as well as the psychology of public attitudes toward a hero figure.

Yeon continued to make animation targeted at adults with dark, controversial themes that brutally and incisively explore human nature and social realism.

His second feature The Fake (2013) critiqued organized religion, as a cult leader swindles rural, uneducated villagers out of their compensation money, while no one believes the local wastrel who discovers the truth (the characters were voiced by Oh Jung-se and Yang Ik-june, who previously starred in Love Is Protein and The King of Pigs).