You have to gradually explore the threats posed by the monster. It can start with something innocuous but unnerving (say a few strange bumps or the sound of chains rattling in the night) and then get harder and harder to ignore.

Assuming the monster has to eat, you could incorporate the bad roommate trope of food going missing. Maybe the protagonist had a big fancy steak they were excited to eat and when they look in the fridge, it's gone. Then later on, you can establish the lib has been actively feeding the monster.

I guess the culmination would be that a person goes missing. Like the protagonist's partner spent the night there, and allegedly had to leave early in the morning, but when the protagonist wakes up, one of their partner's shoes is still in the house.

The monster is a good stand-in for the external pressures facing people such as food, rent, and living expenses. Your main guy and his lazy roommate could be responsible for feeding the monster and paying its share of the rent.

Your main character could try get the monster removed by calling the landlord, an exterminator, and even the police, but no one wants to help, the almost seem complicit with the monster. This could add some paranoia to the way your main character thinks.

The lazy lib character could be independently wealthy, coming from a rich family or has his own stupid exploitative business that doesn't require him to do anything. He's fine with the monster becausehe can afford to maintain his share without doing anything.

Despite that, the lazy roommate could start giving the monster your main character's food to cover his share. For no other reason than because he can, your main character is at work all day and the roommate wants to feed the monster without doing anything.

As the other commenter said, the monster can start consuming other parts of your main character's life before he goes in to confront it himself, finally being consumed himself.

On the theory side. Start with the communist manifesto and Capitalist Realism. They're short reads and it's easy to find a free PDF online.

Read as much theory as you can from actual books. The thinking goes a lot deeper than you'll get from just comments here and it will help inform your writing on other projects as well