Juliet seems to take the zombie apocalypse and resulting depopulation of San Romero more or less in stride, so it's easy to think there's something wrong with her, but then she tells Nick she's been hunting zombies since she was an infant. She's not sociopathic, she's just used to this.
And the zombies' colorful language towards Juliet can probably be attributed to Swan being angry that she chose Nick over him.
It makes sense that Juliet, despite being The Ditz, apparently has a 3.4 GPA. A number of extracurricular activities require a certain, minimum GPA in order to participate, and she's a cheerleader.
Juliet being set on chasing the present from Cordelia throughout stage 2 makes sense when you realize that zombie hunting supplies probably make up most of her wish list (and indeed, every birthday present from someone except Nick in the good ending is a power-up). She knew full well it was going to be something helpful, hence why she went after it right away instead of prioritizing helping Cordelia (who she also knows is a badass zombie hunter in her own right who can probably handle herself), giving some logic to what otherwise seems like, and is lampshaded by Nick as, Skewed Priorities.
The birthday presents Juliet gets from Sensei Morikawa and her family all make sense considering who they come from:
Morikawa gives her the Chainsaw Dash, which opens the way for new zombie hunting moves and lets her make a lot more progress a lot faster in taking them down. Since he's her teacher (and probably didn't expect he'd be dying soon), this is his way of furthering her training and abilities.
Cordelia gives her a chainsaw upgrade that lets her use it as a gun. Cordelia's a sharpshooter; obviously she'd want Juliet to start learning the techniques of using a firearm.
Rosalind gives her the Nick Popper; a large party popper that fires heads. With how goofy Rosalind is, a giant party popper makes a lot of sense.
Gideon, her dad, gives her the helmet for Nick Shoot. He may be a badass, but he's still her father, and wanted to give her something for safety - likely not intending it to be used on Nick instead.
In the good ending, Nick gets to run around on Morikawa's body, but Morikawa's 77. At best, this is a temporary stay of execution for Nick, and he now gets to deal with all the messy side effects of being an old man.
Maybe, maybe not... Morikawa was stooped and bent and kinda slow due to his age - when not shredding zombies, anyway - while Nick wearing Morikawa's body was just short, not showing any such elder flaws, suggesting Morikawa's restored-and-repurposed body now matches Nick's age, if not necessarily his original stature.
Or maybe Morikawa wanted to make sure that at least his body would be the one getting it on with Juliet, even if he couldn't.
Hell, let's face it- if Juliet can keep Nick's uninfected head alive through magic, there's probably another one that can edit Morikawa's body to match Nick's. Either that, or the Powers That Be were having one last screw-around with Nick and will return him to his real body after they're done.
Juliet talks about having the Nick Popper on her amazon wish list and wanting the helmet for Nick Shoot for awhile. Why would she want these things unless she was intending to behead Nick sometime soon anyway regardless of zombies? Granted, the helmet is useful with having a severed head to stick it on but the Nick Popper is a gun that fires a severed heads!
Is there any indication that the Popper was specifically meant for Nick? Considering the number of severed heads Juliet is likely to come across on a given zombie hunt, a gun that weaponizes them might be a good investment.
There is an amazon product in this world. That Specifically Shoots Severed heads. Shoots. Severed. Heads. one must wonder at the practical application of this thing.
Handheld basketball launcher? Something a sadistic coach would find useful.
Or it was a part of amazon that only zombie hunters could access. In the real world, amazon can display in Japanese, French, and other languages depending on your computer's or your profile's settings. Maybe there's a zombie hunter part of amazon.
It might be both are weapons/accessories utilized to weaponize severed heads that Juliet planned to get and use severed zombie heads as ammo. Having Nick beheaded just means easier access to said ammo.
During Morikawa's explanation of what is needed to be done to stop Killabilly, he mentions that afterwards all the captured souls would be sent back to Rotten World. The problem is, there are only five souls actually from Rotten World, and they already got sent back when Juliet killed them. This means that she inadvertently condemned her town, her friends and everyone else she knew to this universe's hell.
That's only if you assume two things: First, that the zombies you've been fighting retain their original souls. They've probably passed on already, with their bodies being inhabited by the gasses from Rotten World. Think less like Dawn of the Dead (1978), more like Evil Dead. Second, that the Dark Purveyors were sent back when Juliet killed them. They were used as sacrifices in Swan's ritual, so they're still probably binding Killabilly together, along with Swan's own soul and the various zombies from Rotten World.