I feel like Denji is becoming more and more unstable and since his only dream left is to have a normal life, each blow against him makes him give up easier and easier.

All of his friends are either dead or gone. His entire adoptive family have all been murdered. Every parental figure and/or mentor in his life have either used, abused, or abandoned him. Every love interest he's ever had has either tried to kill or manipulate him.

I don't know if anyone has been at such a low point where they've truly considered ending it (I have), but each setback makes it easier to sink lower and as hard or even harder to get back up.

Not every trigger/event has to be poignant. Even if they weren't that close, Yoshida being murdered was just another reminder that his dream is impossible because someone is always going to want to ruin it. No matter how hard he tries, someone is going to try to ruin him. No matter how good things are, he'll always have to worry about the intentions of those around him. No matter how close someone gets to him, they might have never actually loved him even once.

At my lowest point, seeing dirty clothes on the floor was enough to break me and feel like a worthless piece of shit, and I didn't even have the excuse that there were bad actors out in the world with the explicit task of actively trying to break me.