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.
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 wishlist (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 with her techniques and be much 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.
Why do the kids Juliet that is tasked with saving throughout the game turn into even more powerful zombies if she doesn't save them from being turned? And why are there still kids to save if the zombies were all caused by a gas that possesses their bodies? Well, given the "gas" appears to have a supernatural quality that makes it akin to a mass of souls able to possess human bodies, it could just be that some people are naturally more resilient and can resist the possession. And, if turned by excess biting or scratching by other zombies, that spiritual fortitude turns into extra strength in the zombie they're turned into. Which is why a zombie hunter family like Juliet's, Morikawa, Nick, the chopper flier, and all the people in need of rescue aren't turned in the initial gas release. There's just not very many of them compared to the masses of people who were infected right away.
Swan's dialogue bemoaning how Juliet killed all the other Dark Purveyors before the Lewis boss battle. On first listen it sounds like he's being sarcastic or haughty, thinking she could never beat him. But knowing he wants Lewis killed to complete the ritual, it's probably at least partially intentional. To not clue her in that something else is going on, and lead her to not waste Lewis, thus putting the ritual to a halt. It's a very intentional, and well used, case of Bad "Bad Acting".
When discussing the possibility of having babies in the future with Nick despite him being a severed head, Juliet proclaims she can have one made using magic, but this means there would be a 50% chance of it turning out to be a cannibal. This seems like a throwaway quirky joke, but remember that Juliet's is a Zombie Hunter family, and she has knowledge of necromancy. Any magic she used to make a baby would probably be related to the undead. Hence it's a distinct possibility for the infant to have a taste for human flesh.
After fighting Zed, Juliet starts dismembering the other members of The Dark Purveyors. A strategy she likely picked up on from that fight since Zed proved that The Dark Purveyors are a little harder to kill than the average zombies she's used to. She may not be able to kill them straight away but she can at least cripple them to make it harder for them to fight.
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.
Juliet talks about having the Nick Popper on her Amazon wish list and wanting the helmet for Nick Shoot for a while. 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 severed heads!
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.
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.