D&D 5E Need an outsider's perspective here.

[knasser]

One talk with your players to see if their game style matches yours. Do they want to be murder hobos where they always get a get out of jail card because they are the stars? Or do bad deeds cause bad things to them.
Since one player just wants to bring in another pc then the Paladin is in trouble. I see three different ways to settle this. 1. New pc and paladin fights the witch and win. 2 They lose. And the witch becomes a minor villain for a couple episodes. Too many ways to plot this after the lost.
3. They run away and never go back.

The trouble is, at least from my experience and I feel the OP might have a similar mindset to me, you get resentful of having to turn characters into villains just because the players decide they want to fight the NPC. The OP has created an interesting NPC and now they have to put her in a villain-to-be-killed box because their players can't stand to have someone they can't look down on or beat up. I could be projecting from my own experiences - OP can and should correct me if I'm way off the mark. But what they've described has strong echoes from groups I've GM'd for and I've really resented the universe of characters being this cheeseboard for players to choose who should be a villain regardless of the actual role or morals I'd envisioned for that NPC.

 

[Lanliss]

One talk with your players to see if their game style matches yours. Do they want to be murder hobos where they always get a get out of jail card because they are the stars? Or do bad deeds cause bad things to them.
Since one player just wants to bring in another pc then the Paladin is in trouble. I see three different ways to settle this. 1. New pc and paladin fights the witch and win. 2 They lose. And the witch becomes a minor villain for a couple episodes. Too many ways to plot this after the lost.
3. They run away and never go back.

I would go the opposite direction. If they go back with plans to murder the witch they meet new guards, Paladins who have sworn an Oath against a certain being, who the witch is helping to keep sealed away. The seal requires a sacrifice, which is why she has all of these dark souls just laying around, waiting to be used for something good. I would make it clear that she is a good guy, though not necessarily a people person, and that killing her would be wrong. If they kill her anyway, they now have a demon they unleashed, and an order of Paladins out for their heads. I would literally make it so that they have to be either stupid of willfully trying to misunderstand me to think she is a bad guy.

 

[transtemporal]

Remember that PCs basically see evil NPCs as treasure pinatas and the definition of "evil" is usually in proportion to how much they want the gear.

So if the NPC is say, a witch - it's pretty easy to assign them with the evil label which in turn makes it ok to "liberate" their stuff.

This goes on in the real world quite a bit too, lol.

 

[ammulder]

I also have a player who just wants to kick butt. His first reaction to any NPC is to say "I kill him/her" (whether or not that's in his power -- it's just his gut reaction). I can guide him ("you understand your head comes up to the kneecap of that Giant, right?"), but I think if I made him apologize for his rudeness to the NPC, he would just be way too far into not-having-fun-now. So whether or not the witch was in the right, whether or not the players were being reasonable, you may have just gone too far for the one player (the one who now wants to re-roll).

If I were the GM here, I'd be looking for a way to turn this to the PCs' advantage, and give them a clear path to success/reward that ultimately requires working with the witch, making them look smart in hindsight for not going back to kill her. Maybe someone *really bad* comes along, and it turns out there's some advantage to capturing their soul instead of freeing it. Perhaps, for instance, their soul is sworn to a demon, and if they're killed, the demon can do something "even worse" with it (step into it to manifest in the world or whatever). The best/only way out is to get someone to capture the soul, taking the bad guy who presently uses the soul out of play without giving the demon a foothold. Surprise! We know someone who can help us with that. But the soul angle would have to kind of unfold through play, so instead of looking like a desperation maneuver it looks like these interlocking threads were planned all along.