This is a bloodthirsty old man deciding to kill someone who was no longer threatening him and was running away.
i assume that if greer ever hires a lawyer competent enough to keep him away from random reporters in his driveway, that lawyer will try to argue something like that because greer believed this couple had broken into his house multiple times, and this time threw him to the ground and broke his shoulder, that he believed the only way he could be safe was to ensure that they would not return.
the fact that he's a feeble old man who comes across as something like a cross between elwood and getoffmylawn clint eastwood will probably play well with most juries. the fact that he killed the woman instead of the man will not.