Preacher season 4 has finally revealed how Cassidy was first turned into a vampire. Since his chaotic introduction in Preacher's first season, Joseph Gilgun's anarchic, party-loving Cassidy has been somewhat of an enigma. Preacher never fully explores the existence of vampires on Earth, instead presenting them as just another hidden quirk of a mad and unholy world. Their mythology was somewhat expanded in Preacher season 3 with the addition of a vampire fan club that lured Cassidy in with the promise of camaraderie and willing blood donors, and it had already been revealed that the vampire had a son that had already lived past middle age.
For the most part, however, Cassidy's history and motivations have remained a secret, with the character driven purely by his love for Tulip and a lack of any other attachments in life. In the early stages of Preacher season 4, it's perhaps an understatement to say that Cassidy endures a tough time but, strangely, he appears resigned to his fate. Captured by The Grail back in Preacher season 3, Cassidy is rescued by Jesse and Tulip but ultimately refuses to escape, instead opting to return to an endless cycle of torture at the hands of foreskin-obsessed pain expert, Franky Toscani.
In a break from the penile mutilation, Cassidy decides to open up to his angelic cellmate, who immediately senses a backstory coming. The ensuing flashback takes viewers to Ireland in 1916 - a period of great conflict for the country, when Republicans took part in the Easter Rising in an attempt to finally end rule by the British. Known then as Proinsias Cassidy, Gilgun's character is a rebel trying to make his family proud by fighting for his country's independence, but it's painfully obvious that the young man is out of his depth on the battlefield.
Cassidy and his young, scared comrade, Billy, are caught in a bomb blast and the latter loses a foot in the attack. Approached by 2 British soldiers, Cassidy watches on, immobilized by fear, as Billy is brutally murdered. Horrified, separated and on the run, Cassidy is hiding from passing enemies when he finds himself stuck in a marshy swamp. A mysterious figure then rises from the waters, desperately trying to drag Cassidy down into the depths, and after the Irishman fights back, the creature lands a bite on Cassidy's neck. Shamed by both his cowardice and his newfound status as a vampire, Cassidy refuses to return home to his family.
This is more or less the same as Cassidy's origin in the Preacher comic books, with the key difference being that Billy survives the actual Easter Rising conflict and he and Cassidy are both attacked by the swamp creature. Instead of escaping, Cassidy is dragged away by the vampire and never sees Billy again. Why the vampire attacks from a swamp isn't clear, and while bloodsuckers certainly have a place in Irish mythology, there are no direct links to the monster seen in Cassidy's backstory.
Nevertheless, a great deal has been learned about Cassidy as a character, and a reason for his self-destructive behavior has perhaps been provided. After Cassidy chose not to escape from The Grail, Franky Toscani speculated that the vampire felt unworthy of freedom. Viewers might've assumed that this had something to do with his feelings for Tulip and keeping their affair a secret from Jesse, but it now seems that leaving Billy to die is the true source of Cassidy's anguish. As well as explaining why Cassidy chose not to walk free in Preacher's season 4 premiere, this reveal might also account for the vampire's generally carefree attitude and his clear difficulty in forming healthy bonds with others.
Preacher season 4 continues with "Search & Rescue" August 18th on AMC and Amazon Prime.