I hear you, although I think they have all been letting the destiny of Earth control their decision-making since season one (it's their literal destination -- the place they are destined to go). Lee and Adama as well as Kara and Roslin have made this prophecied haven their strategic goal and they've fought and died in the race to get there and to interpret the mystical clues along the way from Pythia/Eye of Jupiter/the god or gods, etc. They've all been willing to set aside life and happiness for the sake of Earth, that's the central story of the series. I see Kara's destiny as a continuation of that long-standing story of humanity's destiny, rather than as a departure from it. Though I'm with you in thinking that her part of the story was incoherently told. Sigh.