That's something we have discussed here before, along with similar theories about what might have happened that day. Although, I doubt Els would have "ordered" Zaina's death in any direct way, since she is just an intelligence officer, and she is not a commander in charge of army units. Although she could have "authorized" it to a degree, and feel some level of responsibility or guilt over how it ended up affecting the MC. Also, we're still not sure about the nature of how Zaina was killed, either. A "cleanup" op sounds like the most likely scenario, but I wouldn't be shocked if even information suggesting that scheme turned out to be a mislead in the end.
Regardless of what it is specifically, it's almost a lock that there is some past connection between Els and the MC that will be revealed in future chapters. I mean, one of the very first scenes alone with her has major flags for this very thing:
mc "I suppose I'm just a bit surprised that you have a photo of me at all."
e "Really now? Why is that? We're friends."
mc "We are. I guess I just didn't know if you considered us that close."
mc "We hang out, sure. But I think I can count on one hand the number of times we've done so without others."
...
e "If you wanted to spend more time together, all you had to do was ask."
Classic Els. She doesn't give an answer, and instead she distracts you with an enticing offer to get to know her better. I mean, it's pretty clear to me that she knew quite a bit about the MC before they ever met in person, and the only question remaining is how deep that connection runs.