Historically, self-defence is considered to be in response to an immediate attack. In the case of a woman who kills her partner, it's important to understand that once the immediate attack occurs, it's usually too late. In some cases, then, a woman kills her partner in his sleep, but after she's tried to escape or call police numerous times. It was really self-defence, because there was no other way for her to escape alive.
The courts have made room for recognizing self-defence in those cases, but it's always somewhat risky, as my colleague explained. It's like rolling the dice: risk life imprisonment—the sentence for murder—or plead guilty to a lesser charge. That's where plea bargaining comes into play. Arguing self-defence can fail. In that case, working out a compromise or receiving a lesser sentence isn't an option; in a murder case, the only option is life in prison.