Colum MacKenzie wanted Jamie to be his heir, and lead the men at Leoch. When he disapproved of Jamie’s marriage to a sassenach, he tried to have her killed as a witch.
Dougal MacKenzie wanted to be Colum’s heir - so he tried to kill Jamie. When that didn’t work, he brokered Jamie’s marriage to a sassenach, to remove him from the line of succession. Then realized he’d rather have the sassenach for himself.
But in both cases, the sassenach finally claimed the upper hand. By earning the brothers’ respect for her healing, Colum turned to her for a painless death. By making a foolish decision to threaten her, Dougal lost his life at her - and his nephew’s - hand.
Jocasta MacKenzie Cameron wanted Jamie to be her heir. Took him off guard. Did not quite share the respect for the sassenach wife’s healing - for her views on slavery had poisoned Jamie’s mind.
But it is precisely because of the love and respect and trust that Jamie has in Claire, that he sees Jocasta’s offer for what it is. A gilded prison, built on the backs of people whose lives are very similar to what Jamie experienced himself at Helwater.
He cannot, in good conscience, become the leader that Jocasta - or, for that matter, Colum and Dougal - want him to be. He needs to be - must be - his own man, who lives by his own deeply-held principles.
A decision that would undoubtedly make Ellen MacKenzie Fraser incredibly proud - not just for standing up to the family that all but abandoned her, but also for never failing to do the right thing. With the love and support of his sassenach wife - much like Ellen had the undying support of her outlander husband, Brian Fraser - at his side.