There is an npc/sidekick in my campaign who was a soldier in a war that happened many years ago when he was 28. Half orcs invaded his village and killed his family and he moved to the town of Phandilian. Eighteen years before the main story of the campaign a mind flayer convinces him to summon a horrible evil dragoon demi-god from the nine hells in order to try and save his family. The npc is gullible and ate the story up. Fast forward eighteen years. The sidekick  joins the party on many quests and he becomes their favorite character in the campaign. Later on the sidekick poisons one of the party members and runs away without mentioning anything (He did it because the BBEG was tricking him and making false promises). I don't know how to end all this satisfyingly. I  don't know if a redemption arc will work.

The BBEG didn't make false promises, but rewarded the sidekick: The sidekick is now a struggling minor lord, trying to keep his peasants happy while also raising coin enough to fulfill his obligation to his liege. That's what you get for wishing to be lord of a castle. 

The PC's may never discover this - or they may find out, and go there to confront him. In which case he'll be 'oh, yea, well - I did do that, and I'm really sorry. Also, can you help me please?'

Honestly, a redemption arc would work really well for a sidekick something like, right before the BBEG battle,        BBEG: (NPCs name) Kill them.       

Sidekick: *Pulls out shortsword* Yes, master.

BBEG: See, your little companion has turned to-   

Sidekick: *stabs BBEG*     

BBEG shocked: I see how it is. *Stabs sidekick with greatsword.*   

Sidekick to party: I'm *cough* sorry.

I would look to your players to see how they react when they see him again. You can decide if the npc feels repentant or whether he tries to justify his actions, but if the party is all ready to treat him as a bad guy lieutenant, I would give them that adversarial relationship and build the npc into a formidable rival. 

If the party, on the other hand, seems to want to turn him away from the bbeg's service, I would try to find a way that makes doing so dramatic and difficult, and ultimately more rewarding at the end. 

You got to read your players on that one. Do whatever is going to be their satisfaction for a conclusion to that situation.


But i'd say more or less an NPC that backstabs that party is generally a bad idea in my POV. You as GM are world God, you are also all the NPCs.

As player i can't see the GM being tricked by the GM, if you see what i'm say'n here.
That NPC didn't get tricked, or did you honestly give the NPC a DC check they had a reasonable chance to beat?
So did your NPC get a reasonable to beat check and fail, or did you the GM make a decision about abusing the players' trust in this NPC?

And then really from the player POV, unless you open rolled that kind of situaiotn for them...
That will still just be you the GM that had plans to backstab the players.
And even if you gave ok odds to beat it, were the players just subject to a save or get screwed roll you just made up or had you been laying out some earlier player agency option to detect and prevent this backstabbing?
It's kind of a big distinction between players had agency to avoid something vs. players were just subject to a roll they had no agency over at all, that the GM just made up basiaclly.

This is why it is dangerous to run certain kinds of normally common story lines, on the players. Because in the end all such story lines would not work on the PCs, if they took the game to be the GM is their adversary and they can't be trusted, so no NPC gets any meaningful trust ever.
When God rug yoinks you, what agency did you have to avoid that?
Players need to be able to trust their GM, not treat them as an adversarial zero sum competitor.

This is why as GM i won't and see it as can't, rug yoink/backstab the players.
Any such story line at my table is made obvious to the PCs upfront. The NPC is one they know will try to backstab them. And likely just have no choice but to work with, and so start planning how to cope with the inevitable backstab when it arrives, cause they know it in advance.

The game system Forged in the Dark makes an explicit rule on the GM that we are not allowed to "alter the deal" or backstab players once agreements with NPCs are made.
And that game is about being the baddies, so they knew baddies backstab as a common story beat. But that's a real world problem at the table for GM/player trust relationships.

Players must be able to trust the GM. It is a hard and weird dichotomy to work out. We have to be able to make the world dangerous to them, without us as the GM being seen as their adversary. We should be seen as their fan who wishes to see them succeed, even though we are also creating all the obstacles in the way of that success.

they find a peice of his clothes snagged on a bush. is he dead? undead? alive? captured? no one knows.....

Perhaps set up a battle with the BBEG and his forces that is overwhelming to the party. This NPC swoops in and does something dramatic, like cause a cave-in or something, that prematurely ends the battle and separates the overwhelmed party from the BBEG, saving their lives. He can then explain to the party why he did what he did.

And for something really dramatic, that NPC's intervention could cost him his life. Give him the obligatory death speech to explain himself. And now the party has something more to be pissed off at the BBEG about.