Why was Celia on trial for murder and what legal issues did her defense team face during the trial?

What is known is that a man named Robert Newsom purchased her as property sometime in the 1850’s and began serially raping her perhaps on the day of the transaction. While the particularities of those details are forever destined to remain murky, the fact that Celia kills him and ultimately faces execution for that—crime, for lack of a better word—is crystal clear. The defence team try to argue that Celia acted out of self-defence.