It's interesting how Sae's backstory is actually only the third we've received so far, if I recall correctly. We obviously get Koharu's, of course, and then we get Ena's, which intentionally has a lot of parallels to Koharu's but in a more straightforward tragic sense, her being a person who didn't have much love in her life compared to Koharu missing her chance for love due to her not noticing.
Sae's glimpse of her backstory seems to break the pattern once more; it seems as though she was killed due to what I read as a bullying victim snapping and cutting her throat in the bathroom, which is triggered some sort of PTSD when Ena starts viciously fighting her.
Obviously this is a job of sorts for the cupids, so a disconnect will be inevitable in some degree, but Sae being so uncaring of the actual humans she's pairing up that she let someone die just so she didn't have to put her in her calculations.... crazy. I wonder how the story will weigh the moralities of the characters. Does Sae deserve hell?