An “in-law suite” is different from renting a room. It generally has its own entrance, and a devoted kitchen and bathroom. It’s an entire 1-bedroom apartment built into the house or property (often above a garage, for example).
And it’s not slang, it’s a term that’s been used since the early 1900s, and as the term suggests, it has historically been used to be able to care for elderly parents (so they can maintain their independence while still living with family). It’s not like you can sell an in-law suite separately, and selling one’s house while a parent doesn’t need that and expecting to not only buy another house and having one available with an in-law suite when a parent does need it is a pretty extreme expectation. So it really does come down to rent the room or leave it empty.
And plenty of people want that kind of temporary rental, if they don’t want to be tied to a particular spot for long or don’t want the responsibility of owning.
I mean, dude had tons of feelings. They just got channeled into fury, because he was a Furyan (and had a ton of trauma).
Also, it’s easy to be a serial killer when you’re constantly surrounded by absolute assholes trying to kill you. The guy wasn’t out there sacrificing babies and families for “line must go up.”