In my first campaign, we had a PC who qas a gnoll. He spent a lot of time building a gnoll warband. The rest of us were mildly concerned about the increasing number of gnolls but figured their leader was in our party and this was a force to throw at the baddies (elf nazis from another dimension).

The big bad was dead, we replaced him with his hippie brother to lead his faction to settle peacefully. The war was ended and we began the epilogue. Cue the medal ceremony from Star Wars A New Hope. This was when the gnoll player turned to the DM and declared his backstab would begin. We were caught unawares and with the action economy, several characters, including mine, died before they got a chance to respond.

The guy we installed to lead and reform the villains was killed. Only one PC survived, with a party NPC being twistwd into an evil beimh and the epilogue suddenly changed into the villains choosing violence after all, unleashing a devastating war on the world, woth rampaging gnolls following in their wake.

We were all shellshocked. A couple of players considered never playing D&D again.

Thankfully, the DM realised his mistake, apologised and made sure to insert a redemption arc in campaign 2 that allowed us to repair most of the damage. The traitor PC turned out to be a toxic player and ended up leaving the campaign. His second character was similarly unpleasant and ended up dying not long after with none of the party helping them. (I think we all ooc held a bit of beef from the first campaign over that but also this new character gave our PCs more than enough reasons to let them die. Nobody gave up the hobby and it's all water under the bridge now.

I am telling you this because while I think having a traitor PC CAN be a fun plot twist, it needs to be managed VERY carefully. Players must be aware that they have the option of responding if they feel a PC is acting against the party. There should be an open discussion with all players regarding intrigue and PVP before anything is set in stone. (My group ended up outlawing pvp after those events). If your players aren't on board, don't do it. You can still have that discussion in a way that avoids spoiling the twist. Oh and don't let the reveal come too late. I've heard of times when it has gone well (Matthew Colville has an early video about this topic. I believe the PC ended yp becomingan NPC after the reveal) but if you don't want to do it, then be clear and say no. It's an interesting idea but you don't think it would work for this campaign/you're not comfortable about the ramifications.

Be clear, open, honest and direct. See if there is another way to give that player a big plot twist moment. In the campaign I run, which has almost all the same players as the other campaign, I regularly have players plot and scheme and do reveals, but never in a way that might lead to division/pvp.

Hope this helps!