Gilles de Rais

A nobleman from 15th Century France. As a leader in the French army, he fought alongside Joan of Arc to put an end to the Hundred Years War, and was hailed as a hero who saved his country. But soon after he returned to his territory, he began kidnapping, raping, and killing young boys one after another, eventually coming to be feared as the villain “Bluebeard.”
However, it was all-too-common in that era for nobles to disregard the human rights of commoners and treat the populations of their territories as disposable livestock. So, under normal circumstances even killing them en masse shouldn't have been treated as anything more serious than animal abuse. But unfortunately for Gilles, he had an enormous amount of land holdings at the time, far exceeding those held by the Duke of Brittany, and enough to be regarded as a threat by the king himself. On top of that, due to his mind-boggling wastefulness in his private life, his public finances were almost entirely depleted, and there were apprehensions that he may try to sell his land to enemy nations. As a result, he was condemned and executed for his crimes as a pretext for seizing control of his assets.
As for Gilles' relation to sorcery, he turned to alchemy in an attempt to relieve his financial difficulties, and eventually strayed down a dark path that led him to try summoning a demon. But while it's certainly true that he participated in the ceremony, the actual magus was his friend François Prelati—Gilles himself was nothing more than a patron. As such, he is not a Heroic Spirit that qualifies for the Caster class. Or rather, he is not even a proper Heroic Spirit at all. The fact that Ryuunosuke's bogus ritual ended up actually completing a summoning sorcery was, honestly, more or less a total fluke. In that sense, the “Caster” Gilles de Rais is just as much of an irregular Servant as the “Assassin” Sasaki Kojirou.

Fate/Zero material: Fate/Zero Encylopedia

The man who went on a rampage as a character in Fate/Zero, and whose unexpected reappearance shocked everyone, even myself. As everyone already knows, he is the man known as Blue Beard, Gilles de Rais.
In this work, he appears as a Servant… or rather, as a guest character to break Ruler's heart.
However, since Shakespeare summoned the true Gilles, he changed sides after being persuaded by Ruler. Though Gilles de Rais is someone suited for breaking Ruler's heart, he is the least suitable person to turn into Ruler's enemy. Shakespeare knew that risk and yet still enthusiastically wrote the script that way “because it seemed interesting” to him. This is what you would call reaping what you sow.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia