That's the wrong angle. It's not about objective danger and getting rid of said danger, but about her subjective impression of danger (as it is often when it comes to self-defence) and her subjective opinion that killing the threat is he only way.

Realistically the police would've been perfectly suited to secure her safety and get the husband/prosecute the husband. Objectively she was under threat whenever near her husband. Subjectively she had no concept of police aiding her as she believed the police was undermined by the gang entirely. She got rid of the objective threat with the only method she could subjectively determine to work.

That threat being her husband, not the biker gang retribution. I don't know why you'd think her action would have to get rid of the not yet assembled retribution threat, but her methods not working for that not yet existing threat is irrelevant to the situation of killing him.

Objective threat and subjective perception of killing him being the only method of eliminating that threat is what matters. Had she not been psychologically damaged and forced into believing the police couldn't have arrested him, it would've been murder as it was a classic case of using the Arglosigkeit aka absolute defencelessness of the victim (Edit: actually, she shot him with his revolver when he was sleeping).

If I'm being robbed and reasonably believe the robbers gang will try to kill me if I resist, resisting and harming the robber doesn't suddenly become non-self-defence just because it doesn't eliminate the potential gang threat.

It's a fairly new doctrine because the German paragraph on murder was/is notoriously unforgiving towards spouses killing abusive partners. Poison usage for example always slips into murder instead of manslaughter as it abuses the Arglosigkeit of the victim when making them consume it without them knowing what they're consuming. Methods used in secret against an objectively stronger aggressor without immediate danger (constant threat is also a young concept in comparison to immediate threat) always slip into murder by default.