TL;DR: How can I psychologically justify why relatively normal (i.e., non-sociopathic) human beings that have turned into monsters (like vampires or werewolves) devolve into feral beasts or vicious predators without relying on some innate supernatural biological drive compelling them to harm people or otherwise turn on humans?
I have a story which revolves around a group of monsters in a modern setting, most of which were "turned" from humans in some way. The monsters are incredibly varied in terms of their morality and worldview. Some try their best to fit in and pass amongst humans despite their condition, others basically just devolve into feral beasts eating animals in the woods, whereas others start actively preying on humans and other monsters. The story mostly focuses on the "passing" monsters with the more malevolent ones serving as foils and antagonists. The main draw of the series is meant to be a psychological drama exploring the diverse psychology and worldviews of the characters and the different ways they perceive and deal with their monsterhood.
Some of the rules of the monsters in the setting and their plot reasoning are listed below, so as to make the rest of the question make more sense:
- The monsters more-or-less live hidden among humanity. I.e., typical genre trappings.
- The monsters don't need to prey on humans to survive, in order to allow the monsters much more moral latitude and explore different types of monster tropes. Eating people isn't as unthinkable as it is among humans, but how taboo eating people is varies massively from group to group. I.e., the more sympathetic monsters are horrified and disgusted, whereas the less human ones don't care.
- Becoming a monster does give a person monstrous instincts and can warp their mind a bit, but the person mostly remains the same. Personality shifts can occur but happen due to lived experience post-monster. This is partially due to story themes and partially because the whole point of a psychodrama is to explore someone's thought process.
- The monsters aren't exceedingly long-lived or immortal. Mostly to make them more relatable to the audience.
- There isn't really a huge faction of monster hunters. The monsters are afraid of persecution by humanity but it's not like there's a group dedicated to hunting them down. Mostly because the "monsters being persecuted by monster-hunters" trope has been used so many times it's become cliché and there isn't much one can do to make it fresh.
However, when writing the story I am running into an issue in trying to understand how a relatively normal person could devolve into a barely-human beast squatting in the woods or a monstrous predator. I've tried researching other "monster" stories like the Old World of Darkness and Tokyo Ghoul to see how those settings handled it, and mostly I have come across a few major explanations. However, these don't seem to work with the psychological drama angle of this story for a variety of reasons.
- The monsters become increasingly estranged from humanity due to the monsters being immortal and losing all of their human connections. This is how the Old World of Darkness tended to handle it, and it does make the most sense psychologically. Lose all your connections to humanity and suddenly one's ability to empathize with them becomes difficult. But the monsters aren't immortal here, so the "weight of ages" is hard to pull off.
- The monsters have to eat humans in some way and those that do not sufficiently dehumanize human beings end up starving to death. From what I've read supposedly the Sabbat in Vampire the Masquerade worked along these lines, but it never really got explored due to lore changes. This doesn't work because the monsters here don't have to eat humans, and indeed the story points out on a logistic and biological level a species that must eat humans wouldn't be able to survive.
- There is some kind of biological compulsion involved that makes the former human instantly turn on humanity. However, when this shows up in fiction this is usually used as a justification as to why the audience shouldn't feel morally conflicted about the protagonists staking their former friends. It doesn't work in a setting where the monsters are more sympathetic. I have seen stories make good arguments as to why someone would choose to be a monster than a human. E.g., many monster stories set in the Victorian era point out that for young women becoming a vampire/werewolf/whatever gives them personal freedom and power not available to them as a human, but there's no reason that wouldn't translate to "I like being a monster but I like people too" instead of "I eat people".
- The former-humans decide to become the monsters everyone sees them as after being persecuted or ostracized by their former brethren (see: Carrie). This kind of works but it's hard to see how widespread it could be without monsters being common knowledge, so it doesn't make sense to make it a universal phenomenon among every feral monster.
- The monsters were evil people before becoming monsters. I.e., the "I reject my humanity, Jojo" principle. This seems the easiest to implement and seems fairly straightforward but kind of removes the personal horror aspect from the story. I.e., it seems almost Calvinist (and a bit morally black-and-white) in that only the "bad" people become "bad" monsters. I know there are cases where regular people can be corrupted into enjoying atrocities (e.g., Nazi Germany, the Stanford Prison Experiment), but most of the time that seems to require mass movements and an external system coercing or encouraging people until they start enjoying or rationalizing it. Additionally, I know humans can be quick to dehumanize others, but that usually happens to those outside their in-group, not those within. Humans can clearly be awful but this seems to be the wrong kind of awful to justify what I need.
The problem with many of these explanations is that they rely on some sort of hardline biological compulsion to turn people into monsters, which is terrible for a psychological drama because it removes most of the elements of personal choice. Consider, in a psychodrama about a vampire the audience wants to know why the vampire thinks and acts the way they do (e.g., Interview with a Vampire). If the vampire only does so because they must prey on humans and because they are biologically compelled to hate humankind, that removes all sense of character agency and thus they become less interesting. So I'm trying to avoid that.
Granted, part of the point of the story is to explore the assumptions of monster tropes (and "there's less slavering beasts than you'd think" is a plot point), but the fact is I need the more antagonistic monsters to create conflict, and if I can't understand why my villains and antagonists behave and think the way they do (or more broadly, where they even come from) that creates a problem. Some of the more feral monsters are treated narratively more like dangerous wild animals or man-eating big cats than slasher villains, dangerous but with their own set of priorities and motivations that don't always involve violence, but even getting a human being psychologically to that point seems very difficult to achieve.
So, more broadly, how can I psychologically justify why there would be a general phenomenon where more-or-less psychologically normal people can devolve into monsters after being turned?