There is no point in being a villain when people are in private talks.

People who speak ill of others behind their backs and sow discord. The "Yin people" in this sentence refer to those who speak ill of others behind their backs and sow discord. The "villain" in this sentence refers to those who lack morality and responsibility and only care about their own interests. These people will destroy the harmonious atmosphere of the team or organization, leading to things not going well. So "speak ill of others behind their backs, but a villain can't" means speak ill of others behind their backs.