Personality: Zack often speaks in foul language. He will occasionally swear in the situations he’s in, and will not hesitate even if it is towards Rachel, despite her young age. It is also revealed that Zack never had any education of sort, as hinted in his inability to read, and when characters in the story explain things to him in long terms that makes Zack frustrated and unable to understand unless if it was explained to him in simplest terms. Because of his lack of knowledge and education, when he is put into a situation which requires using the brain to figure something out, he often forces upon Rachel to do the problem solving.
Zack has a mental disability, whether it was something he had since he was born or he developed at some point in life is uncertain. This disability consists of him having a lust to destroying things, even to the means of killing. His blood rushes to a certain point and causes him to lose control of his urges. He will go into a short phase of destroying anything in his way until he’s satisfied or until he rehabilitates.
However, it can be regarded that while Zack is not academically intelligent, he is high in instinctive/survival intelligence, where he relies on his instincts to get him through situations, and can be seen as clever. Which could be how he was able to survive as a child, and up until now. And perhaps can even be described as an animal in a way. Zack is also extremely strong and rather fast, being able to run at rather incredible speed rates, and overall is actually a rather powerful character, which balances out the fact he isn’t that smart. Zack is also VERY brutally honest, as he hates people who lie with a burning passion, and never ever sugarcoats anything he says.
As a child, Zack was seemingly much more quiet and reserved. He was shown to never say a word throughout the manga specifically.
Zack’s first murder: He stayed in a run-down orphanage where children were likely to die, and the trafficking was illegal. His caretakers were rather abusive, and always referred to him as a monster, and even forced him to bury the corpse of a dead child simply because they didn’t want to deal with the smell. Until one day, one of the caretakers left the TV on, where it showed a horror film that included a serial killer. And Zack just-so-happened to stumble upon it while it was on, becoming fascinated with the murderer specifically in the film. And he took inspiration from it, and he murdered his caretakers by mutilating their bodies, and later buried them in the back. And years later, his recent killings became famous, and is currently a wanted criminal.