Those who work with and are in contact with the sacred are especially likely to become its victims. The primary notion is that of contagion. The fear of the contaminated insider abetting an external enemy is crucial to the thinking of many societies, and anthropological analysis of witchcraft can help illuminate how this fear manifested itself in Rome as well. Thus, Philip Mayer in a famous article describes the witch as “The Traitor within the Gates” (1970, 60):
The figure of the witch, clearly enough, embodies those characteristics that society specially disapproves. The values of the witch directly negate the values of society…. However, I think that another or a more particular kind of opposition is also vitally involved. I mean the opposition between “us” and “them.”… The witch is the figure who has turned traitor to his own group. He has secretly taken the wrong side in the basic societal opposition between “us” and “them.” This is what makes him a criminal and not only a sinner.
These remarks cast an important light on the Vestal Virgin. For the Vestal accused of incestum was held to be not only a sinner but a criminal as well, and the worst criminal of all: a traitor-ess. The specifically feminine form is significant. In undoing herself, she has undone Rome. I say “undoing herself” in the same sense as “got herself pregnant.” For a feature, usually unnoticed or unremarked by both ancients and moderns, is the entirely optional presence of a man. The sequence of events is clear: misfortune results in suspicion of unchastity; unchastity implies a seducer; one is occasionally sought and found. While we know the names of several men executed or exiled for having had intercourse with Vestal Virgins and while such a charge clearly might be used for political purposes, Vestals were most often tried for unchastity quite by themselves with no male codefendants, or (just as revealing) the existence of male corespondents was not considered worthy of record. There is no case recorded of a Vestal Virgin suspected or convicted because she was pregnant nor any case where a Vestal was charged with unchastity because she had been raped. Vestals always sinned willingly. It was necessary for them to do so…
Thus, the penetrated Vestal Virgin becomes a witch; that is, when a witch was needed, a Vestal was deemed to have been penetrated. Here we see one of the most frequent uses of witchcraft: to protect other value systems. The failure of sacred ritual can be attributed to witchcraft, specifically to betrayal by those very technicians of the sacred whose duty it was to perform the rituals that protect society. This linking of betrayal and unchastity in the figure of the traitoress (traditura) ran deep in the Roman mind. It is an intimate part of the cultural encyclopedia. It features prominently in myth and mythical history (Horatia and Tarpeia) as well as rhetoric and rhetorical history (Sempronia). It is also enshrined in law, which allows the torture of slaves to provide evidence against their masters only for cases of incestum and for treason.
Holt N. Parker, “Why Were the Vestals Virgins? Or the Chastity of Women and the Safety of the Roman State” (2004)