The thing about Saber, as a character, is that she’s someone who Fucking Commits to whatever role she’s given. And normally that role is “The King of Britain” and which means “To Lead” so she does just that, She Leads to the point of death or deification if this is what it takes. On a sillier note this is why in FGO you get shit like “Evil Saber as Santa” or “Evil Saber as a Maid” because once again if she’s given the role of Santa or Maid or whatever then she’ll be the best fucking santa maid you’ve ever seen.
The role of Salter is “the villain.” And you’d expect it to mean “To Destroy” or “To cause suffering” or stuff like that, but no. The role of Salter is “To Die.”
Salter is The Villain. She’s the dragon in the story. She’s the one who has to be killed for the world to know peace. She’s the obstacle which filters out the weak so only those strong enough to fix shit come through. She’s the villain that makes you a hero by being defeated.
To be clear she doesn’t want to die- she doesn’t resent living or being a villain or anything, but it is her duty to die and king Artoria Motherfucking Pendragon is nothing if not someone who upholds her duties. That’s why she lets Shirou live after he kills Berserker (he earned his right to a proper fight with her) That’s why in Sparks High Liner (when Shirou fights her one by one) she keeps telling him what he needs to do to save Sakura mid-fight, and why she asks him to kill her when she’s down. That’s why she’s disappointed in Shirou if he refuses to kill her. Her duty isn’t in serving someone perfectly or in killing as many as possible. Her duty is to die.
It’s a characterization that has survived all the way to FGO, where her first appearance is guarding a corrupted grail. By doing so she’s stopping you from saving the world, yes, but also everyone else from destroying it. She’s the villain who opposes you, whether you are good or evil, and her duty is to die so you can fulfill your own.