Wild Mass Guessing for Angels of Death.
Reverend Gray is
Jigsaw, or at least part of the Jigsaw Movement at the end of Saw 3D.
Gray's methodology of breaking both Ray and Zack is exactly that of Jigsaw's, but less self-mutilation and more psychological torture. As for Gray's heavy religious undertones, it is comparable to
Wisecrack's analysis of Jigsaw's methods as having basis from Christian theology
and its obsession with Redemption by
Purgatory. The rest of the Floor Masters, such as Daniel, Eddie and Cathy, were, like Jigsaw's apprentices such as Mark Hoffman, similar psychopaths who managed to pass Gray's tests and become Jigsaws themselves.
That explains his
insane durability, able to endure life-threatening injuries (such as slicing himself in defiance of Cathy) and the reason for how almost everyone refer to him as a monster, save for a blind man who adopted him as a son... just like how the original Frankenstein's monster found a sense of family and belonging thanks to a blind man adopting him. And like Frankenstein's Monster, Zack, upon the death of the blind father-figure, went onto a murder-spree taking revenge against humanity. Who knows, the reason for wrapping himself in bandages all the time is to hide his hideous artificial nature.
Rachel is a
Sociopath, and the entire point of the Floors is to make her embrace her Sociopathic nature, and become a full-fledged Floor Master
Most of the tests carried in the floors have verified how Rachel was callous and without empathy, trampling others that were obstacles to her goal: her oath to God, of Zack killing her once they escaped. Eddie was a potential friend to Rachel, but dismissed him as nothing but fodder for Zack to kill. Cathy's punishments were more focused on Zack than on Rachel, but the latter implicitly manipulated the former to slice himself, making a disturbed Cathy go down to the stage where it was open for Rachel to shoot her. Finally, Gray had to use his hallucinogens and Biblical theology to put Rachel into a Witches' Trial, where all the witnesses denounced Rachel as a selfish manipulative witch... that which she remained unrepentant. It took Floor B1, her own floor modeled after her own house, the house where she murdered her own parents, to convert Rachel to the purpose on why she was sent here in the first place. In fact, Rachel designed the deadly traps of Floor B1 herself.
Going from the Jigsaw WMG previously, it can be speculated that Reverend Gray saw in Rachel a brand new heir, a complete psychopath devoid of emotions, but only loyalty to the Justice of God. The other Floor Masters were too flawed. Danny was consumed by Lust; Eddie Prided himself in making the best graveyards; Cathy was a wrathful sinner who manifested her anger issues in the form of torture scenarios. Rachel was none of that, for she sublimated all of her emotions, for the sake of God and loyalty. She would make Reverend Gray's perfect heir.
The existence of Zack, however, screwed everything up. Instead of using her own sociopathic talents to dedicate oneself to the service of God, Rachel instead swore an oath a completely different God, a mortal God, a God that promised to kill her. Rachel made Zack her own God. This scenario was a Spanner in the Works for Reverend Gray's plan to make Rachel his heir, and, it was Zack's very own presence who retaught Rachel the value of emotions. Because of the Unholy Contract Rachel and Zack made for each other, the Reverend's plan was completely destroyed, and soon in the end, he accepted his demise and the destruction of the facility.
