Under normal circumstances, Jaime would never kill his beloved sister.
The valonqar prophecy would be fulfilled, and the self-destructive relationship between the Lannister twins would have finally self-destructed.
He wouldn’t even be the only son of Tywin Lannister to kill the woman he loved in the heat of the moment.
Under circumstances in which his beloved sister lied when she promised him a child that he could actually acknowledge as a father rather than an uncle… well, his restraint could finally break.