In History: Cotton Mather studied at Harvard and worked as a minister in Boston. He studied witchcraft in depth and even wrote a book, titled The Wonders of the Invisible World. The book attempts to examine witchcraft from an unbiased perspective and suggests that only confessions, not other forms of evidence, should be used to convict a witch.
On Salem: A well-educated man who has been sent to Salem from Boston, Cotton Mather (Seth Gabel) is meant to oversee the witch trials. When he thinks his father, Increase Mather, has gotten too wild with his own witch-hunting tactics, Cotton believes he has no choice but to kill him. And so he does, thereby saving Mary Sibley.