Castle was used by Sarah

I can't shake off the thought that Sarah used Castle time and time again. First she hid the fact that she was pregnant when she went to bed with Castle, who fell in love with her and would take care of her and her baby. Second, she made Castle collaborate with a communist in order to smuggle her out of South Africa. She must have known that Castle would have to betray the MI6 after collaborating with a communist agent. Third, she wouldn't give Castle his own child. It was strange why they wouldn't have their own child but just keep her illegitimate child. Over all Castle betrayed his country and his "firm" in order to save Sarah and her child even though Castle was not a communist and didn't even like the Soviet Union.