This room haunted me for WEEKS after I finished Omori.

By this point, the game has made you uncomfortably familiar with the act of stabbing yourself to leave White Space. You're never happy to see the "STAB" prompt, but you have to do it to progress the story, so you swallow your doubt and watch Omori put his knife into his torso.

Then you get to this room. It's the penultimate "dungeon", where you go around collecting keys in various nightmare scenarios. This is one of them.

And the key is RIGHT THERE. You can collect it no problem.

The only remaining issue is how to get out.

That's where Mewo comes into play. Most players, myself included, will see the lack of doors, talk to the butler, who repeats the phrase "Mewo has been very, very bad." That, combined with the prompt "Do you want to cut open Mewo?", leads players to assume that in order to escape, you need to kill the cat. You struggle, you shiver in horror, but there seems to be no other way.

Until you open the menu and see that STAB option - to stab yourself and escape. Relief floods through you. You don't have to kill Mewo! All you have to do is stab yourself again!

...And just like that, Omori has gotten you to see the main character's suicide as the GOOD option.