Fringe theory.

Admittedly, my knowledge of Gilles de Rais' crime is more passing interest than "expert" but from all I have read, the veracity of his guilt is not in doubt among academic circles. What usually is contested is the actual number of victims and the exact nature of his crimes, but even then that's usually not heavily debated. While there have been a number of revisionists that have attempted to clear Gilles name (everyone from Voltaire to Aleister Crowley), none has offered an extremely convincing set of evidence dispelling Gilles' own testimony and the numerous testimonies of witnesses who lost children to him (in his book on the subject, The Trial of Gilles De Rais, Georges Bataille estimates that there may have over 70 witnesses testifying against Gilles), or the fact that rumors about Gilles' unusual predilections had existed for years prior to his actual trial. The famous "retrial" of Gilles took place in 1994 and was organized by Gilbert Prouteau, who at the time was writing a biography of Gilles. At the risk of sounding critical, it was very much in Prouteau's interest to have a big song and dance about Gilles' innocence right on the cusp of releasing a book about the man alleging that he was innocent. The woman responsible for the Gilles de Rais Was Innocent website is an author, not a historian, and even Prouteau's famous retrial group was mostly politicians and lawyers, not medieval historians. In her essay on Gilles de Rais in The Hundred Years War: Part III, Elena Odio notes that

On the other hand, the seemingly overwhelming evidence against him has convinced most who have studied the case closely to agree with the court that the Breton nobleman was one of the most prolific serial killers in history...

I think she is entirely correct in that statement. In my passing assessment, Gilles de Rais was a deeply troubled man. The loss of both his parents at a young age (he actually witnessed his father's apparently horrific death in a hunting accident) and an extensive and brutal career in military service probably contributed to a less than sound mind. Not to mention the stress that would've come with the responsibilities of organizing an important noble house at a young age. He had a terrible temper from his youth onwards as well and was incredibly impetuous and, unsurprisingly, horrible with handling money which he spent like it was going out of style. Even the manner in which Gilles was eventually brought before a court (he barged into a church in a rage and kidnapping a weeping priest from the middle of a service for supposedly cheating him in a business deal) illustrates a man who was of less than sound mind. He definitely had an obsession in his later life with the occult, specifically alchemy, perhaps as a means to regain his flagging fortunes.

His defenders usually cite the lack of distinct physical evidence and the sudden nature of Gilles confession while at trial. He truly went from defiant on the stand to completely repentant in a night. The aformentioned Elena Odio points out that Gilles had grown increasingly obsessed with Christianity and redemption as he had aged and it was well within the bounds of reason that, realizing just how much trouble he was in and that he would be unable to brashly and angrily get out of it, he would've gone with the path of repentance and guilt rather than continue his defiance for no good reason. Similarly, the sudden change in tact was not out of character for a man renowned for his impetuosity and spontaneity. And, of course, there is the fact that no evidence, circumstantial or otherwise, exists to support the idea that he was tortured. As for physical evidence, in a time without today's modern crime scene investigation technology and the ability to extensively search for evidence of the crime, I think it is less pertinent. While I am perfectly willing to believe that certain aspects of Gilles testimony where either the fanciful delusions of a troubled mind or intentional inserts from a court looking to ham up their case against him (talking to devils or whatever), I think the core of the crimes remains solid and believable.

The reason I poked at Gilles' possible mental health issues is to get at a key concept I think people seem to forget where he is concerned: Occam's Razor. The simplest explanation is often the best, and which is simpler: That Gilles de Rais, a powerful nobleman with a history of family tragedy, strange behaviors, sudden rages, and a penchant for violence (not to mention a relatively itinerant lifestyle), was perfectly positioned to be a serial killer and thusly was one? Or that Gilles de Rais, powerful nobleman and comrade of Joan of Arc, was framed in an elaborate conspiracy between church officials and other noblemen (or even the Inquisition, as some have posited) that nobody questioned at all in order to steal land that they were already buying from the increasingly impoverished Gilles on the cheap? I think the former carries more weight because it is more believable. Gilles has all the hallmarks of a serial killer, it's not a stretch to believe he actually was one, and I think most historians of the time period and subject would agree with me in that assessment.

Sources:

Bluebeard Has His Day in Court, The New York Times, http://www.nytimes.com/1992/11/17/world/bluebeard-has-his-day-in-court-not-guilty.html?mcubz=3

The Trial of Gilles De Rais by Georges Bataille, 1991

Gilles De Rais: History's First Serial Killer?, Encyclopedia Brittanica Online, https://www.britannica.com/spotlight/gilles-de-rais-historys-first-serial-killer

Gilles De Rais by Elena Odio in *The Hundred Years War (Part III): Further Considerations, 2013