The Stig wrote:
It's also worth pointing out that Starbuck is hardly a good judge of character, so relying on her word that the fleet was 'less safe' with Cain gone doesn't hold much water.
Cain was intended to be what Adama would have become without Roslin. It's all there in the miniseries and the coup in Season 1: he was set to leave the civilians to die at Ragnar so he could go out in a blaze of glory, fighting a hopeless battle. Given his tendencies towards authoritarianism, he could have very easily gone down the path of raiding civilian vessels for military needs.
As others have mentioned, BSG was about what it meant to be 'worthy' of survival. How much can and should we give up of ourselves in the face of horror and disaster? Cain failed her test soundly from the show's POV. It takes a fairly slanted reading to get anywhere close to admiration for her or her actions. Her treatment of her former lover in and of itself is more than enough evidence that Cain was motivated not by military need but petty revenge for being duped by a skinjob.
She became ruthless before the personal betrayal. So I'm inclined to believe it didn't have a part in it. "Worthy of survival"...that was in reference to Adama killing Cain. Cain was about to kill Adama, but she did not either. When she saw how successful a collabotartive effort between the two ships could be in taking out the Cylons, she chose not to take Adama out. That indicates it was more about taking out the Cylons than any personal motives.