The original script for TUC had Saavik in place of Valeris, however Nick Meyer, Leonard Nimoy and Gene Roddenberry agreed that it would be wrong for Saavik to be a traitor, since she was regarded as a fan favourite, not only that but also her motivations would be complicated...for example:
Funny, I had head that the reason for Valeris instead of Saavik was that "the producers" balked at having the same character played by three different actresses in four films. Robin Curtis was unavailable (though I can't imagine why, as the imdb shows her making a single TV movie and appearing on General Hospital that year), and Kirsty Alley was either unavailable, had put on more weight than the producers thought acceptable, or they just balked at the idea of going back to her for some other reason (including money, as she'd gone from unknown in Wrath of Khan to having been on one of the highest rated shows in TV for 3 years by Undiscovered Country).
Just rumor, but that's what I'd heard.
I can see Saavik having much the same motivations as Valeris.
But try this one: we have seen it is possible for Vulcans to disagree. Each would believe their position to be the logical one, but each would have based their position on certain assumptions.
Spock extending the olive branch is based on a belief that kindness pays off in the end, and that absolutely everyone is better to have as a friend than an enemy. And perhaps most importantly, a belief in the ability of people to change, and that exposure to humans breeds friendliness towards them.
Saavik/Valeris believes the Klingons are incapable of meaningful fundamental change in a useful time frame. That is, the peace cannot last: the Klingons will eventually attack the Federation again. That being the case, it is best to destroy them now, when they are weak, rather than let them rebuild before fighting them. Better still if they will destroy themselves and you don't have to lift a finger.
Both believe their position to be the only logical course of action. One says "hope" and the other says "safety".
After all, the very best bad guys think that
they are the hero of the picture.