Agatha's last name is Penderghast; that, her resemblance to Norman, and the common ability to see the dead all-but-confirm that they're related. How did the ritual of reading her "a bedtime story" get started? Who else would read a girl's favorite story at her grave but her family? Her mother, at least, survived her and loved her very much. The ritual must have begun as a memorial service, an act of love.
The whole reason why that one zombie stood there staring at Mitch's car as it runs him over and Judge Hopkins petrified look to his zombie brethren as he hung onto the top of the speeding van for dear afterlife? The judge and the six others didn't have cars back than, not to mention the fastest thing they saw was no doubt horse-drawn carriages. Not to mention the six nameless zombies baffled look on modern day Blithe Hollow.
Another subtle moment happens in the town hall. After Norman sees Agatha's trial through a vision, he wakes up to find the zombies standing close by, looking timid and ashamed. Though the movie doesn't make it too obvious, it's strongly implied Agatha actually killed the Judge and the puritans when she lost it at the end of Norman's vision, which would also explain why they were buried in the same clothes they wore at the trial. The Judge attempts to approach Norman, but backs off when Norman yells at them to keep away. A minute later Norman stands up quickly upon realizing what the book actually does and all the zombies back up as if afraid of him. If they did all die in said vision, then this becomes more meaningful when you realize Norman is standing in the exact spot Agatha was before she killed them all.
Regardless of the exact moment that Aggie's curse kicked in, it still makes perfect sense that they were buried in the same clothes they wore to the trial. People in that time period and culture tended to have a very Limited Wardrobe. They most likely owned only one set of fancy/"Sunday" clothing each, which of course they would have worn to be witnesses in a formal witch trial...and also for their burials.
Norman tells Neil in his backyard that ghosts only stick around if they have things left to do or died in a sudden or bad way. Aggie was put on witch trial, murdered, AND could see the dead like Norman, which is why she stuck around and was able to possess her raging powers near the end.
"I wanted everyone to see how rotten they were!" Of course you did, Agatha! After all, you brought them back as shuffling corpses with most of their skin rotted off! It'd be funny if it weren't so terrifying.
The story in the book is Sleeping Beauty, keeping Aggie asleep for three hundred years because of the(fear of the) witch's curse, until Norman battles his way through a forest of thorns to reach her, and then... she falls asleep on his shoulder. PG movie, and they are related. The witch in Sleeping Beauty is also uninvited to the princess' christening out of fear, too, which might be part of it.
It's specifically mentioned that the witch was sentenced to hanging. The sort of hanging that breaks the neck quickly wasn't developed until the mid-1800s. Poor Agatha would have suffered for upwards of twenty minutes, or even longer, considering her low body mass relative to an adult.
Uncle Prenderghast was only one in a long line of people who performed the "bedtime story" ritual, and all of them could see ghosts. Norman's family has a ghost-seeing gene of some sort, and Uncle Pendergast wanted Norman to follow the tradition and stay in town. Agatha could see ghosts. Agatha's pain and rage got stronger with time despite the ritual. Between the shared last name and similar appearance, Agatha is related to Norman and his family. In conclusion, Agatha's family has been unwittingly prolonging her suffering for centuries!
Due to his own ability to see and communicate with the dead, Uncle Penderghast ended up as a pariah among both the community and his own family. If the events of the movie didn't go as they did, Norman would've likely ended up the same.
Worse, it's likely that most if not every person who had the abilities of Norman and his uncle were probably local outcasts as well, which means that despite the treatment they were given, they were the ones responsible for keeping everyone safe from the witch's curse. Meanwhile, the townsfolk themselves continued to make things worse by passing on the legacy of Agatha with an insulting and utterly false caricature of her.
Why didn't the zombies, in all their moaning and limping glory, try to act less scary when attempting to get Norman to listen to them? Their spirits are actually trapped inside their decayed corpses, meaning their minds, motor functions and vocal chords are damaged beyond anything healthy. So even if they still have human level intelligence, they're stuck acting like the undead ..."Make you suffer" indeed...
Actually, outside of just being zombies, they didn't do anything scary. After Norman's vision of Agatha they could talk just fine. It was just the heat of the moment. The suffering comes from the townspeople tearing them apart. Also, not passing on and all that.
Minor as far as a zombie movie goes, but the way Norman's parents were fighting at the beginning, if Norman hadnít been proven right and his dad hadn't gotten off his high horse (and if his mom had any self respect) a divorce would have been right around the corner.
Given that Agatha and Norman are related and can both see ghosts does this mean that if events had turned out badly, or Norman had become bitter as a result of his treatment the supernatural powers she used to kill seven people and curse them to eternal undeath would have awakened in Norman?
And the exact same thing that happened to the Judge and his ilk probably would have happened again if the townsfolk had succeeded in lynching Norman. Then they'd have two angry spirits on their hands.
Doubles as a Sequel Hook, but you notice how the movie says that ghosts stay behind they have something left to do? Well, Aggie's unfinished business was to get revenge for her death, and the movie ends with a hint that she moved on, although it doesn't say that giving up your unfinished business will free you from this world. So for all we know Aggie might still be out there and fate might be pushing her to complete her revenge if she wants to see her mother again. Only real bright side is that Norman would have a new friend, but still.
Agatha didn't stay for revenge, she stayed after she died because people didn't understand her, so she need someone to do so. Since Norman did it, she could move on.
The zombies' first acts upon rising from the grave are to scream, howl, and flail around - terrifying Norman and Alvin in the process, and coming off as much more dangerous than they actually are. Well, considering it turns out they've retained their full human consciousness, personalities, and emotions while trapped in the rotting shells of their bodies...it's surprising they manage to stop screaming and calm down in time to follow Norman at all.