Did I miss something plot wise, why did aragaki leave originally?

[bulldog0890]

they keep dancing around this without saying why, and i'm curious if i either missed something or they mention it later. something went horribly wrong apparently, and also he's stuck taking meds and now in his linked episode he told me he's gonna die? but like...huh? i feel like things were missed

[Mario from 64]

Have you already gotten to October 4th?

Basically, Shinji couldn't entirely control his Persona, and one night two years, it went berserk and killed Ken's mother. Shinjiro feels deep regret for this and it's why he left SEES and started taking the suppressants. The suppressants gradually tear away at the consumer's body, so Shinjiro knew that he was going to die from having used them for so long.