As per usual, my players shouldn't be reading this post.
Ok so 3 real world years ago in the very first encounter of our campaign I introduced a 10 year old townsfolk NPC child who, in the midst of a goblin attack discovered she had arcane talent and fought back against the invaders. The campaign has progressed well,, the party is reaching level 18 and the NPC (now an adult, a level 16 sorcerer, and all but married to one of the PCs) has occasionally joined them on adventures in a henchman capacity where her bloodline has proven to have a connection to and power over the monstrous minions of the big bad.
Now the group has been joking for years that this particular NPC will turn out to be the campaign final boss so when I started riffing on this particular idea last adventure it was pretty epic. Unbeknownst to the party the big bad kidnapped and cloned her a bunch of times, the party had a big final battle against a lesser Jabberwock and a couple of casters, one who they presumed to be this NPC but it turned out to just be a clone.
Upon rescuing the original most of the party think things are back to normal, with the potential exception of dealing with her clones cropping up in the future.
One of my PCs however doesn't see it this way. The party rogue, the usually lovable loose cannon, has decided that the NPC is too much of a liability in the face of the Big Bad's looming apocalypse and has decided to kill her and/or trap her soul such that she can't escape or be resurrected. The player wants this plan to be kept secret from the other PCs both IC and OOC.
Now the rogue is the powerhouse of the party, I have zero doubts about his statistical ability to push this plan through. However this is really going to cramp my style as I wrap up this campaign, plus upset both the PCs and their players.
These are the options I've come up with so far and I'm not entirely happy with any of them.
-Tell the player not to despite he's convinced "it's what my character would do"
-Let him proceed with the plan and give the party tools to find out what he's been up to so they can fix things, party turns on each other.
-Let him proceed and have a pivotal NPC simply vanish from the campaign.
-Give the NPC a sudden boost in power such that she can unexpectedly ward of the attack.
-Have the NPC grow wise to the threat on her life and vanish for a few sessions only to come back and make a preemptive strike against the rogue.
Any ideas as to how to best handle this for the enjoyment of all.