Who the Parasite really was:

Rating: 8/10 (9 February 2020)

We find the prosperous Park family to be the occupants of a slick, scando-sheek home in Seoul, Korea.
Mr Park is a fantastic provider for his family; in quintessential Korean fashion; as a tech entrepreneur. His wife and two children seem not to be in want of anymore than they already have. Despite this, his family are precariously vulnerable to social exploitation. His youngest son, who 'saw a ghost', is now subject to seizures. His daughter is a hopeless English language student who only sees her tutor classes as an opportunity to get a boyfriend. Finally his wife is highly impressionable and not fully competent to tackle the issues of her two children.
In addition to whatever jobs he has created at work, Mr Park employers a driver, housemaid and English tutor for himself, his daughter and his wife respectively. All of these seem to be happy in their employment.
The Kims are another family of four but, for reasons we are not sure of, live in very basic accommodation in a poor part of the city. None of the family have solid jobs but it is not explained whether this is due to fecklessness or sheer bad luck despite best efforts. Lack of ability seems unlikely in their case as we see that they are very clever, albeit in a Machiavellian way. For example, they are happier to trick their way into getting paid for doing a bad job then actually folding the pizza boxes correctly.
The English tutor at the Parks home recommends to the son of the Kims to temporarily replace him as tutor to the daughter of the Parks on the one condition that he leaves her alone for him to ask out when he returns.
This soon snowballs into all members of the Kim family shamelessly exploiting the Park family and their employees.
Their crimes are as follows:

Forging an official university document and using it to gain employment.
Exploiting and encouraging the mis-diagnosed mental illness of a young boy.
Getting two people fired by destroying their reputations.
Stealing food and alcohol.
Murdering Mr Park because he didn't like the smell of a man who:

Lived underground for 4 years
Had a barbecue sword of meat sticking out of him
Was dead

The Parks weren't perfect but nowhere did they claim to be. What they were was a respectable family who contributed to society in a positive manner.
The Kims were a clever but immoral family who continually made bad decisions at key moments, bringing their downfall on themselves.
Unfortunately the parasite was so entrenched that once its downfall became the downfall of the host.