So...I guess the most likely scenario here is that weapon that Himeko used had some sort of poison to temporarily put Chikane in a near death state right? I vaguely remember seeing someone mention years ago on a form, a point in the webnovel version (which I never actually read) where Chikane is trying to get back to the Island to stop a marriage ceremony so I'm guessing that we are just before that part and It'll probably be revealed soon that It's more than just "Himeko was secretly evil" because that's always how it goes with these 2.
Actually, if the manga followed the novel, we would be quite far away from the part where Chikane gets to the island, but since the manga skipped over and changed a lot of things from the web novel, we are a lot closer to that. Also, one of the differences is that in the web novel, Himeko is getting married to Souma.
There is a reason to what Himeko did, but it doesn’t mean that she had always the same objective.