The title's misleading. There are no zombies and no supernatural elements. To be honest, that would have been easier to watch. I've seen this film called a black comedy, but personally I didn't connect with it.
Its main character is Nami (Takiuchi Kumi). She had a messed-up childhood and became a messed-up adult. As a child, it seems likely that she's acting up to try to get attention. She deliberately pushes over a glass of milk at the dinner table (not so bad), then attacks a classmate with a mop without provocation and then cheerfully volunteers her guilt when the teachers come running. She turns into a sullen creature who watches the shopping channel in the dark, philosophises about loneliness and spends lots of money on goods she doesn't even open.
There's despair, parental abandonment and suicide. That's the first twelve minutes.
As an adult, though, she embraces her loneliness. She loves it. She's rich as a result of inheritance from that suicide, so she doesn't need to work and instead spends all her time on what she calls Solitarian-watching. She stalks people who've gone crazy due to loneliness. She might climb on a roof with binoculars, or perhaps break into their stinking garbage houses. If she finds that they've died, she'll be delighted and take a selfie with the corpse.
Her "Solitarians" include:
(a) an old woman who does nothing all day, living in despair because her only son died.
(b) an obnoxious man who marches through the streets, shoving people aside.
(c) a crazy street bloke who feeds pigeons and gets bitey if anyone tries to bully him. (The film's teasing you with its title here. You'll be wondering if that bloke's a zombie.)
Misery makes her happy. However she'll explode with rage if some do-gooding Christians start coaxing one of her targets out of his grumpy shell of self-hatred.
Other people have enjoyed this film, calling it "quirky" (eh?!) and praising its darkness. Alas, for me, it didn't work. I didn't care. I simply wasn't interested in Nami. She's a horrible person, obviously, and I saw no reason to care about anything she did. (Her childhood was bad, yes, but that's not an excuse.) What's more, we're seeing the world through a Nami-filter and so it's hard to feel close even to the people who are nice (the Bible-bashers) or rude-but-lonely (Takashi Sasano as Shiomi). It's worth pointing out that there are strong parallels between the former and Nami's mother, who was even more of a bleeding heart and as a result destroyed her family. Besides, we're talking about someone who won't stop ringing your doorbell and asking if you'd like to read the Bible together.
Needless to say, this film is messed up. It's doing dark things with an interesting cocktail of themes, from religion, altruism and what might be virtue-signalling to everything that's going on in Nami's head. It's not the genre you'd expect from the title, but it could be argued to take a non-supernatural horror turn.
It wasn't for me, alas. However there are plenty of people who disagree with me, so you might too.