Kassandra only grabs the staff after the vision reveals the purpose of the Remnant of Argon, meaning everyone knows what it is and what it does.

Kassandra isn't attempting to use it as a weapon of mass destruction, she's trying to use the spirits of Argonians within the Remnant to forcibly connect herself to the Hist Trees, as she's been fairly open about her motivations for collecting relics in the past quests.

TBH I'm still trying to figure out why the Blackguards (who work for her) are kidnapping the Cyrodillic Collections people (who also work for her). Kidnapping the Nagas makes some sense, as they're generally not helpful to outsiders, but at that point the Collections people had no reason not to tell her everything they knew if she just asked. And kidnapping them raises the odds they'll get killed or lost in the swamp trying to escape, thus completely denying her any information they might have had.

I found it weird that the Remnant of Argon damage potential's is never concretely explained, but as an object able to store souls with a strong connection to the Hist, its power is pretty clear. The staff alone gave Kassandra powerful combat abilities, a miregaunt mechsuit, and powers over the Hist.

In her dialogue during the final fight with her, she says:

Kassandra: "I can hear the Hist! Finally, I can hear it!"
...
Kassandra: "The Hist's power is mine to command!"

https://en.uesp.net/wiki/Online:Kassandra

She might have been "working with" the Blackguards, but that doesn't mean that she trusts them -- the same way she's "working with" the player. She was able to use everyone to get what she wants.

Then why the big panic to stop her?
...
when did Kassandra suggest that she might aim for any sort of "dark purpose" at all?

When she stole an ancient relic, paralyzed a group of people, and threatened to kill people on her way to seek out even more power. Doesn't exactly suggest someone who had selfless intentions for the relic, and of course she wouldn't reveal this nature early on.

Chasing Kassandra was a way of chasing the staff to the Remnant of Argon (the characters say this) - I think stopping her became important once her violence and betrayal became clear, and was necessary to actually acquire the Remnant of Argon.

I'm so disappointed in the murkmire main quest. It would be better if they developed the idea of a prologue with a necromancer and skull-horcrux. ZOS's mainquest could be realized in two side quests: injure Cassandra and artifact (root-visper viladge story)

WhiteCoatSyndrome wrote: »

TBH I'm still trying to figure out why the Blackguards (who work for her) are kidnapping the Cyrodillic Collections people (who also work for her). Kidnapping the Nagas makes some sense, as they're generally not helpful to outsiders, but at that point the Collections people had no reason not to tell her everything they knew if she just asked. And kidnapping them raises the odds they'll get killed or lost in the swamp trying to escape, thus completely denying her any information they might have had.

You can correct me if I am mistaken, but I see it in that's way:
Kassandra wasn't a big boss, most likely she worked under Whiptail or even for Drakeeh the Unchained as legal fence of contraband argonian artifacts via Cyrodillic Collections and Margus Derius most likely was part of this in such way they rise funds for Blackguards, but first such artifacts must be found.
In such way she employed adventurers and archeologist for doing work on what dumb Blackguards mercenary incapable.
After items are found Blackguards just plunder cleared ruins or mug adventurers for loot that's why there was such staff turnover in Cyrodillic Collections.
Someday Kassandra found information about Remnant of Argon and decided use it for self, to ultimately achieve connection to Hist, I do not think that's Whiptail or Drakeeh was aware about her plans, most likely she lied them, so after she run Whiptail decided capture Cyrodillic Collections members and squeeze information out of them about actual Kassandra plans, as we can see he have own plans to resurrect buried Sithis worshiping and become his champion that why he need Remnant of Argon to power ritual, not sure if Drakeeh aware of this also, but I see connection of Whiptail plans and Shuxaltsei resurrection in Teeth of Sithis most likely all those Blackguards vampires around was under Whiptail command, and he involved in resurrection of last pre Duskfall Sithis priestess most likely she manipulated him.