Over the course of The Undiscovered Country, Lieutenant Valeris goes from being Spock's right-hand-lady to a political terrorist. It's not such a good look.
Not only did Spock sponsor Valeris, a fellow Vulcan, when she was in Starfleet Academy, but he's also training her to be his replacement on Enterprise. That's huge. It's like a young rapper getting a co-sign from Jay Z.
So why does she betray him? Well, according to Valeris, it's not a betrayal at all:
VALERIS: Klingons cannot be trusted. Sir, you said so yourself. They killed your son. Did you not wish Gorkon dead? [...] And you were right. They conspired with us to assassinate their own Chancellor. How trustworthy can they be? Valeris is a walking example of how fear can prevent us from making the "undiscovered country" of the future a better place for everyone. Influenced by the prejudices of Kirk and his generation, Valeris sincerely believes that she's doing the right thing by fighting against peace.