Warning! Major spoilers ahead for Chilling Adventures of Sabrina season 4.
Season 4 of Chilling Adventures of Sabrina killed off both versions of the titular character by its end, but it’s still unclear what caused Sabrina Morningstar’s death. Sabrina Morningstar is the result of a time paradox created at the end of season 3, which saw Sabrina Spellman (the original witch) going back in time to save the world from the apocalypse. Sabrina released her past self from a hellish imprisonment and, instead of closing the time loop like she was supposed to, decided to keep it open. This time paradox allowed Sabrina Morningstar to rule over the nine circles of Hell while Sabrina Spellman went back to Earth to enjoy her days as a student of Baxter High School.
Naturally, the existence of two Sabrinas wasn’t going to fly. Morningstar’s presence upset the natural order of the cosmos and the archangel Metatron arrived to force the two Sabrinas to merge. While they both couldn’t stand the idea of becoming one person again, they could no longer occupy the same realm. To prevent worlds from colliding, the Queen of Hell was sent to an alternate Earth. When that world starts to be consumed by the Void, an eldritch terror that brings about the end of all things, Sabrina and Salem, the talking cat, team up to send Sabrina Morningstar back through the mirror portal to warn her doppelgänger of the threat. However, while Sabrina was completely fine on her way into the mirror, she didn’t make it out the other side alive and died shortly thereafter.
Why? One could argue that Sabrina Morningstar was caught between existence and the utter nothingness that was the Void. After all, the teenage witch only managed to get through the portal at the last second and the Void was already sweeping through her Earth before zapping it out of existence altogether. It’s possible that Sabrina had been touched by the Void’s destructive force just as she was halfway through the portal. It would explain why she succumbed to the nothingness of death by the time she’d finally made it back to Sabrina Spellman; the eldritch terrors were cosmically powerful.
Conversely, it’s possible that Sabrina going through the mirror portal is what ultimately killed her. After all, passing through worlds wasn’t a usual occurrence, not even for Sabrina. The portal to send Sabrina Morningstar through to the alternate reality was created by powerful magic and may have only been a one-way ticket to the other side. She risked a lot to get back through the portal to Earth, but it likely did a lot of damage to her body on her way back through.
Immediately after her return, several cuts are visible on Sabrina’s face that aren’t there before she jumps back through. She obviously sustained them on her way through the magical barrier separating the worlds. The idea that the mirror portal is what ultimately killed Sabrina Morningstar in Chilling Adventures of Sabrina season 4 is evidenced by the fact that Salem lay dead next to her in the aftermath. Sabrina had grabbed hold of the talking cat right before they entered the magical door, but it seems he died before she was able to deliver her message to Sabrina Spellman.