Does the word "chief" mean someone was the worst or the most vile?
No. In Scripture, "chief" denotes the most important, head, or primary person or role. Paul was called the chief speaker because he was the most important of the two, not because he was worst or vile. Likewise, Publius was the chief man of the island, meaning he was the head or governor, not a wicked man.
Les explains that when the pagans at Lystra called Paul Mercurius, they did so because he was the chief speaker, meaning he was the most important of the two, not the worst or most vile, and not because he used awful language. Later, in Acts 28:7, Publius is called the chief man of the island, and Les clarifies this does not mean he was wicked; it means he was the head honcho, probably the governor, the chief of that island. So the word "chief" carries the idea of primary rank or leadership, not moral depravity. Les also urges comparing Scripture with Scripture so the Word is not twisted to mean something entirely different.