How does Roger Chillingworth, as a villain, enhance the meaning of The Scarlet Letter?
Interesting question, as some would argue that in this novel Chillingworth is described in terms that put him beyond the description of a mere "villain", yet others would say that such harsh descriptions are unfair.
Chillingworth is often presented in this novel as a demonic figure rather than a credible human being in his own right, and this is perhaps largely a reflection of the way that the other main characters perceive him. Hawthorne puns the old term for a doctor, a "leech", to aid in depicting the physician as a parasite who is literally sucking the life and vital energies out of the minister in revenge for committing adultery with his wife. Consider the following quote which describes Chillingworth's reaction to discovering the self-made scarlet letter on Dimmesdale's breast:
Had a man seen old Roger Chillingworth, at that moment of his ecstasy, he would have had no need to ask how Satan comports himself when a precious human soul is lost to heaven and won into his kingdom.
Clearly, Chillingworth is described in explicitly demonic terms.
He is similar to Dimmesdale in that both live under concealment, and Chillingworth of course goes so far to assume a false name to protect his identity. However, the comparisons stop there. For, instead of putting his faith in God, he takes on himself a God-like role, feeling he has the right to pry into the secrets of another man's soul:
So Roger Chillingworth... strove to go deep into his patient's bosom, delving among his principles, prying into his recollections, and probing everything with a cautious touch, like a treasure-seeker in a dark cavern.
He is clearly a godless practitioner of early modern science who seeks power through knowledge. But, interestingly, when he loses his host, his parasitical existence soon terminates.
However, in spite of the way he is presented as a grotesque and diabolic caricature, he does show self-awareness in his recognition that he was wrong to have married a young and attractive woman. However, this insight does not result in any acts of compassion - it only drives him on in his desire to achieve revenge.