The Role of King James I in Defining Witches

King James I
Without James VI of Scotland, later to become James I of England, it is an open question whether the witch hunts of the 17th and 18th centuries would have reached the apogee they did.
For some brief background, we can point to the Witchcraft Act of 1563 in Scotland; this act predated James' birth by 3 years, but certainly provided the foundation for the English Witchcraft Act of 1604, which followed Scotland's lead in making witchcraft in all its forms punishable by death.
The Witchcraft Act of 1604 itself followed two notable events in James' life. The first was his marriage to Anne of Denmark, and his exposure to the Danish court, which was at the time obsessed with witch hunts. (Smith, 81) The combination of the influence of the Danish court, the storms which beset Anne's attempt to join James in Scotland and which beset the couple returning from Denmark after James took it upon himself to fetch her together likely laid the groundwork for his own fascination with witchcraft.
This is probably where he was first exposed to Reginald Scot's Discouerie, the refutation of which was a driving motivator for James to write his Daemonologie in 1597! He considered Scot so dangerous, in fact, that he called the Englishman out in his preface, saying that his work was, in part, "against the damnable opinions of two principally in our age, wherof the one called SCOT an Englishman, is not ashamed in publike print to deny, that ther can be such a thing as Witch-craft."
Therein James had "proven" that witchcraft was real and a threat to mankind (using the Bible as an authority and final proof on the matter). The Daemonologie also made the case for witches as women, and gave instruction on how to find a witch out. The Daemonologie is constructed as a dialogue, with Philomathes taking the view of the skeptic, and Epistemon taking the role of the expert who "proves" the case against witchcraft.
Chapter V of Book Two makes this plain, when Philomathes asks, "What can be the cause that there are twentie women giuen to that craft, where ther is one man?" Epistemon's answer is, effectively, the basis for the Malleus Maleficarum, which itself laid the groundwork for the witch hunts to follow: "The reason is easie, for as that sexe is frailer then man is, so is it easier to be intrapped in these grosse snares of the Deuill, as was ouer well proued to be true, by the Serpents deceiuing of _Eua_ at the beginning, which makes him the homelier with that sexe sensine."
Translated, James' argument is that women are frailer, weaker, and more prone to the Devil's lures. Thus, the case is made for women as witches. The men who turn to witchcraft are painted as more feminine than masculine in their weaknesses.
It is also in the Daemonologie that we see the basis for outlawing all witchcraft, rather than just the malevolent forms of it, when James/Epistemon says that the cure for diseases of witchcraft is "Onelie by earnest prayer to GOD, by amendement of their liues, and by sharp persewing euerie one, according to his calling of these instrumentes of Sathan, whose punishment to the death will be a salutarie sacrifice for the patient."
In these two brief passages of Book Two, we see James' case for witch hunts as valid; since the Bible proves that witches exist, denying their existence is tantamount to blasphemy. Furthermore, we have a case for women as the primary proprietesses of witchcraft, and the condemnation of the practisers of the craft to death.
Linder, Douglas. UMKC School of Law, "A Brief History of Witchcraft Persecutions before Salem." 
Last modified 2005.

James I, King of England. Daemonologie in forme of a dialogue. Edinburgh, 1597

Larner, Christina, James VI and I and Witchcraft, in Smith, Alan G.R., ed., The Reign of James VI and I (New York: St. Martin's Press, 1973)