Best I can tell, a player is making statements about an NPC that the OP (GM) judges to be inappropriate. It is not clear why you (the GM) think these statements are inappropriate. It is clear that you have become attached to this NPC, and have a certain amount of resentment toward players who make unfair statements about them.
Let Them Think What They Want
It is not within your authority to dictate what a player or a PC thinks about, or says about, an NPC. It is within your authority to make the NPC respond to PC behavior in whatever manner makes sense for that character. More importantly, only you have this authority. No player can make their statements about your character true without your say-so.
Use It to Your Advantage
More to the point, the fact that your players have vehement opinions about your NPCs is in itself a good thing. It means you've made a memorable NPC that the players want to react strongly about. You can leverage this toward a memorable campaign, especially if you use this NPC as a villain.
The player has certain preconceived notions about your NPC. That's fine. How much fun would it be if you decided to subvert their expectations? If they think the NPC is evil, what if the NPC is secretly working toward a good purpose, e.g. protecting a group of unarmed civilians, or leading a resistance against the real Big Bad.
Head Canon
You mention that the player keeps referencing back to facts that occurred in a test campaign, which were changed for the current campaign. You can talk to the player and tell them why you changed the NPC's traits, and how you want the NPC to be characterized.
But it's difficult to let go of past information. After all, all of you spent some hours of your lives establishing these facts. For the slate to be wiped clean, in whole or in part, invalidates some of their time spent. It's like a reboot of a movie series. You can't help but think of these "new" characters using the old characters as a reference. For example, when Vampire: the Requiem came out, it was very difficult for many to think of the "new" clans which used the same names as the old ones, without remembering their old characters.
So, the point here is just be understanding of the difficulty in wiping the slate clean.
Don't Become Attached to NPCs
Your NPCs lives are forfeit. Protecting one of them can lead to a situation of unfair adjudication for a GM, e.g. when they become upset that the PCs don't like their NPC, or have succeeded in killing the NPC. NPCs should serve to move the story forward, not be put on a pedestal. Follow the principle of Be A Fan of the PCs. This game is about the them, not your favorite NPCs.
Squick
If torture is not fun for you (makes you feel uncomfortable), it's in your authority to take it off the table as a PC option. It can be the sort of thing that happens elsewhere in the world, but is not done to, or by, the PCs. Or it can be the kind of thing that does happen, just doesn't ever happen "on camera". Ideally, this is a conversation between everyone at the table, GM and players together.
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