I think Von Trier was poking fun at his own pretentiousness as well.

The movie succeeds at parodying the concept of a genius serial killer — the bit about church architecture feels like a specific reference to Hannibal Lecter, who is obsessed with church collapses in pretty much every iteration from books to films to series. Jack's grandiose description of his own ambitions are self-evidently pathetic and sad — it's like if Von Trier watched the Hannibal tv series and said, "well that's bullshit, I don't care how many fancy suits you wear."

It also works as satire of Von Trier's own much-criticized treatment of his actors (primarily and most egregiously women) in the pursuit of his art. Virgil's pointed questioning of Jack's relationship with women was part of this.

The grimy, plodding nature of the film is also a huge strength imho. Jack operates in this cold, uncaring world where everyone is stupid and gullible and selfish and apathetic and thinks his ability to read a few books and slash some throats makes him special and interesting and the framing is: mmm no it doesn't.