The entire manga is in Satou's mind.
While a cliché idea, this does explain a few aspects of the world like how nearly every other character introduced is a deeply disturbed individual, but considerably worse than Satou herself. It could also make some sense to believe that Satou may have only initially seen Shio in a missing person report and latched onto her creating a life where she was convinced that she was in love with her and that all of the conflict in the manga is simply a figment of her mental unstableness.
Satou will die.
The manga/anime make it clear that no matter how pathetic Satou's upbringing was, there is the likelihood that she would be too far gone for rehabilitation. The only way I could possibly see Satou dying is either from the events of the first episode where she and Shio are about to jump off the building, or Asahi would be forced to kill her, especially when he figures out that she killed Shouko.
Or — alternatively— everyone dies.
Naturally a dark series such as Happy Sugar Life
can only end with no resolutions for any of the characters involved.
1) Satou and Shio die together after jumping off the building.
2) Mitsuboshi either dies in a scuffle with Satou or maybe from suicide given how Shio's Aunt was forcing herself on him. Given his traumatic experience with his original boss, the latter is more likely.
3) Asahi would die in his confrontation with Satou. Maybe Asahi gets to finally see Shio again before meeting his untimely end.
4) Satou's Aunt could end up getting murdered by Mitsuboshi in self-defense. Whether she ends the series killed or alive and badly injured, she probably wouldn't care regardless. She might end up getting killed during when she is Incarcerated by a inmate.
5) Daichi could try to exploit Satou again only to be merked by her.
6) Yunna could possibly succumb to the poison.
7) The Manger possibly ended up killing herself after her defeat.
Shio doesn't exist.
A recent interpretation of the anime and manga series suggests that Shio may not actually be a real character at all. Rather, she represents pureness which is the main motivator for characters such as Mitsuboshi and Satou being obsessed with her. Asahi compares Shio to the moon, which in folklore is known to be associated with driving people insane. Hence the term "lunacy."