Once Upon A Time shows that living in a fairy tale isn't always a dream come true, but in the end, it is a world where good almost always triumphs and consequences are rarely final. There were certainly some heartbreaking moments on the show, but the stakes were rarely a matter of life and death for every character.
Slashers have similar rules to fairy tales when it comes to morality, with good eventually triumphing over evil, but that never comes without a lot of bloodshed first. If the characters from Once Upon A Time were stripped of their powers and thrown into the horror genre, most wouldn't last long.
Zelena Mills
Traditionally, those with the strongest bonds to the final girl last the longest in a horror movie, which is not a good sign for Zelena. Despite being one of the best characters introduced after season 1 and eventually helping out the heroes, she isn't close enough to the main characters to last long.
Her death would probably be early on and gruesome, likely before all of the other characters have banded together for safety. Her death may help push the other characters to stay in a group, but it probably wouldn't be very memorable.
Belle French
Belle is the weakest main character on Once Upon a Time, not having magic or much in the way of fighting skills. Her forte is intelligence, and after that, compassion. Neither is particularly helpful against a killer, but they might mean she would fight long enough to leave behind an invaluable clue as to how to stop the killer.
Because of her caring nature, her death would be painful for other characters and audiences alike, and would likely be one of the first kills, as she is an easy target.
Rumplestiltskin/ Mr. Gold
Rumplestiltskin is a survivor, but he is also selfish and struggles to work well in groups, both of which can be a character's downfall in a slasher film. No matter how strong an individual is, going off on their own is almost always a bad idea.
Rumple would probably try to track down the killer alone after Belle's death and disappear for the majority of the movie, only to be found later on in a gruesome jump scare a la Annie Phillips from Friday the 13th.
Granny Lucas
Granny Lucas is tricky to judge because, despite her age, she is a fighter. With her crossbow at the ready, Granny could put a serious dent in a killer that came against her in a fair fight.
However, fair fights are uncommon in horror movies, and Granny is a strong enough center of the community to warrant her being an early kill, maybe at the end of the first act, where her death would motivate the rest of the townspeople to come together in the hopes of finding and defeating her killer.
Captain Hook/ Killian Jones
Captain Hook has an advantage over other former villains because he is well-integrated into the Charming family. This means he will likely last a lot longer than those who are out on their own.
Hook knows how to put up a good fight, which would keep him alive longer, but other characters are more deserving of spotlight moments, which means his death would probably come as a surprise, rather than being a drawn-out chase or fight sequence.
Emma Swan
Emma may be the savior, but that doesn't mean she would make a good survivor. Emma tends to do things on her own and has a self-sacrificial nature that would prove to be her downfall. While she was often viewed as Once Upon A Time's protagonist, she wasn't always the hero, and wouldn't be the final girl in the horror movie version.
She would probably get a brilliant fight scene, dealing a serious blow against the killer and giving others the chance to get away, but Emma doesn't have quite enough of the final-girl spirit to survive until the end of the movie.
Prince Charming/ David Nolan
Charming is a capable fighter, but he doesn't have the fighting instinct that other characters frequently display on Once Upon a Time. Because he is so important to his family, his death would probably serve as an impact kill later on in the movie, meant to make the other characters feel hopeless or at least be distracted enough to slip up.
His death would either be in front of his family, to show their immediate horror or offscreen, making for a terrifying body find during the third act.
Henry Mills
Henry would survive longer than expected in a horror movie because he has so many people dedicated to protecting him. The rules of horror typically show that a character embodying hope as much as Henry does has to survive at least until the second act, to give audiences an unlikely protagonist to root for.
Unfortunately, Henry has few skills of his own. This means that once those protecting him are taken down or otherwise occupied, Henry will be an easy, and heartbreaking, kill.
Regina Mills
Regina is a strong fighter and knows how to work with the Charming family. She would also fight with everything in her to keep Henry safe, which means she would probably spend the majority of the film with the large group, keeping safe by staying with others. Unfortunately, despite having one of the show's best character arcs, she is also the most morally grey character, and traditionally only the purely good characters get to survive.
Because of that, Regina would probably get to put up a good fight against the killer, but she would not survive until the end of the movie.
Snow White/Mary Margaret Blanchard
Snow White is the only character with the combined survival skills and spirit of goodness to become a final girl. By slasher rules, the final girl should embody purity and goodness, both of which Snow has in spades.
With all of that said, Snow would be able to survive the horror movie and defeat the killer, before mourning the loss of everyone close to her.