Owning a house and owning a person are 2 different topics. One is admirable and the other is barbaric. Kind of saddened you thought to compare the two.
Trashy doesn't want to elaborate too much on his statement, but I can try to detail what he might have meant.
His initial statement is the following.
An analogy I'd like to make is landlords. Landlords fundamentally are evil. They make a living off of leeching of those who cannot afford their own housing - which makes for arguably one of the most vulnerable layers of society.
What constitutes as evil in his opinion is illustrated by "leeching of those who cannot afford their own housing".
From that, the distilled core concept of evilness is the unfair perception of widening the gap between two impermeable groups: the exploiter and the exploited.
It is perceived as unfair because of the political and economical context is designed to protect that gap. Falling into the exploited category of the population leaves you at the mercy of the exploiter.
Let's decline this idea in several examples, trying to put the commonalities together.
Slavery
Exploited: you work for your master, you get nothing
Exploiter: you reap the fruits of all of your slave's work
Enforcing system: laws, slavery magic, etc
Class mobility: None
Corporate slave
Exploited: you work for your employer, you get a 10$ raise after 10 years of loyal work
Exploiter: you treat yourself with a company-bought lamborghini with your employee's hard sweat because why not
Enforcing system: laws, capitalism, stock market, banking system, money printing and the money lifecycle (you're better off sucking the dick of whoever get the money freshly baked from the printing press than being at the end of the chain like the one who actually produce wealth like food or items)
Class mobility: Start a business with an enormous initial capital, and don't fail.
Landlord
Exploited: you can't afford your own house. You have to rent. Each year, you have no choice but to cough up 1/10th of the value of your rented space. After ten years, you own of course nothing.
Exploiter: you get money for existing. You can buy more real estate and drive the price up to widen the gap if you feel inclined so
Enforcing system: laws, capitalism, banking system
Class mobility: Suck your banker's dick to get a good loan. Pray Blackrock hasn't bought your entire city yet
Colonization
Exploited: grow chocolate, coffee, tea or make bricks. Waste all your potential by being converted into "human capital". Get a few dollar per month (little enough so that you can't invest into your future). If you have kids, send them to school. Once they graduate, send them over to rich countries, so they can become slaves with bigger wages. See your land's natural resources getting sucked dry.
Exploiter: get some tea, don't elaborate and leave
Enforcing system: wars, assassination of local politicians to push the greedy ones who are willing to sell their country to you in exchange for authority
Class mobility: Turn your country into such an apocalyptic shitshow that no value can be extracted
If you live in an unfair system, it's hardly your fault for trying to get in the better side of the world. Once you're fucked, you're fucked.
In conclusion, Trashy's "landlords are evil" is a concise summary of those ideas. But I'd like to expand that landlords themselves aren't inherently evil. I'd say it's more the combination of the concept of landlord in the context of our particular economical system.
As such, if a protagonist decides that slavery is bad and they should destroy that unfair system, it would be as unhinged as someone IRL trying to beat up landlords and tear down capitalism.
You don't see that often.
Freeing a slave, would be like illegal occupation of a vacant house I guess lol