Hmmm, there aren't that many great options for sabers. They're all mostly heroic and loyal.

If I were to choose though, Ibuki maybe? I can see her betraying others, although that feels more like her doing it on a whim than a calculated decision.

Dioscori twins were jerks in lb5 but I don't know much about them other than that so not sure if they're the best choice.

And lastly, I think that Yamanami Keisuke could fit into this, as he was skilled in military tactics, and he left the Shinsengumi. (although it wasn't really much of a betrayal)

Also, who are the other servants that you've picked?

Squirrelly eh?

I got your squirrels right here!

Weirdly fits?

I dunno. Otherwise JK Saber (Suzika Gozen).

Why not make your own characters from scratch?

For example, since you were talking about digging down rabbit holes, why not get the White Rabbit from Wonderland to be a character whose goal's to become real and who reveals a jealous rage against another Servant for being based on a real person or something? ┐(￣ヮ￣)┌

How is this 'squirrel' attitude play?

Because if we talk about eccentricity alone, Astolfo easily fits the bill.

He's type who prioritizes his Master well-being, if he likes his Master. He'd have no issue ditching alliance he joined to if he likes his Master and notice the alliance do more harm than good to his Master.

Jason in comparison, is basically a jerk and dreamer who aspired to be everyone's leader.

He's at clash with his Master if his Master doesn't want to follow his lead.

His betrayal in Fate also mostly driven by how he considered other people within alliance doesn't fit his goal anymore rather than actually planning to betray them from start.

If you wanted Saber within FGO that would planned to betray the alliance once there's no more benefit, Yamanami and Saito would do better since they're historically used to underhanded job.

I wanted to suggest Mordred, but she's Saber Face or Lancelot

1. Some heroic Sabers can betray their alliance when betrayal better suits their heroic value. Siegfried in F/A is a good example: He killed himself and basically left his master defenceless in the middle of a war, just so that he can perform his heroic act of saving a homunculus.

2. Generally speaking, Pre-Edo Japanese are more open to betrayals. So you might want to dig a bit around Nobu and her friends/enemies.

3. People who had done this before. Not only Saito mentioned above. Yagyu was a politician, and d'Eon was a spy, they must had betrayed lots of people in their life. Although not implemented as Saber in the game, Lv Bu is confirmed to have the potential to be a Saber(as well as Archer, Lancer, and Rider) and did betrayed a lot in his lifetime. So that makes sense too.