Raynor's oath to kill Kerrigan wasn't wasted. As DarthYam said above, Raynor is
clearly shocked and upset when Zeratul tells him Kerrigan has to live. But when Raynor sees Zeratul's memories he begins ti understand why he shouldn't kill her. Furthermore, later he meets Valerian, who has the way to revert Kerrigan back to her human form, make her the woman she was before becoming Queen of Blades, who killed Fenix and committed all these atrocities. And Raynor
knows Sarah and Queen of Blades are NOT the same, in fact, she outright tells him this when she defeats his forces shortly after he rebirth. Metzen also addressed this matter on BlizzCon and confirmed that Valerian's offer changed everything for Raynor: now he can not just destroy the Queen of Blades, but bring the woman he loves back. IMHO Raynor's speech before the final mission was actually cool. Cliché, but cool. At least it was appropriate. The thing I didn't like, however, was Warfield bragging about how he held the line while Raynor hid under rock, which is an extremely outrageous thing to say, considering the fact that Raynor helped to save the universe from the Overmind. But that's a flaw of Warfield's initial attitude rather than story.