A brief primer on self-defense and the use of deadly force:
(Not legal advice, yadda yadda)

Deadly force is generally justified if an adversary commits a hostile act (uses deadly force or force that reasonable people believe would cause grievous bodily harm), or... https://twitter.com/ScottAdamsSays/status/1277996552120762368

Displays hostile intent (intention to use deadly force as defined above).

Three components to hostile intend:
Ability
Opportunity
Intent

Ability: the adversary has the capability to cause death or GBH. Gun, knife, crow bar, baseball bat, size/training

Opportunity: the adversary has the ability to gain the access required by the capability. For example, adversary armed with a knife within 30 feet = opportunity. Adversary armed with a knife on other side of 12-foot chain link fence with no gate near by, probably no opportunity.

Intent: the adversary through their words or actions displays the intent to cause death or GBH.

If all three are met, you have the right (and in some cases the obligation) to defend yourself and others using deadly force.

** You do not have to wait for a hostile act **

You are not required to let the bad guy try to kill you first.

There are steps you can use to try to de-escalate before the hostile intent triangle is met:

verbal warnings
soft controls
hard controls
non-lethal force

But, and this is important, once the adversary displays hostile intent, you can (should) skip all of these.

In the case of the couple defending their home and selves against the mob:

Capability: a large group with or without unknown weapons. Check.
Opportunity: it is reasonable to believe that a mob can easily break into a home. Check.

Intent: reports indicate that before the couple grabbed their weapons, they received verbal death threats. Check.

Hostile intent triangle met. Couple showed good restraint in not using deadly force.

Another example:

Crowds mobbing vehicles in intersections, pounding on windows, threatening drivers, and in some cases shooting vehicles.

Capability: A mob with/without weapons has the capability to break out windows on a vehicle and cause death/GBH. Check.

Opportunity: In many cases, the mobs create the opportunity by surrounding the vehicle and preventing its safe passage. Check.

Intent: threatening words/gestures, hitting vehicle with deadly weapons, brandishing and/or firing of weapons. Check.

Hostile act/intent met. Deadly force authorized in self defense. Yes, this includes running over the assailants.

Next concept: Necessity and proportionality.

Necessity is met by a hostile act or hostile intent as discussed above.

Proportionality is widely misunderstood to mean you can't bring a gun to a knife fight. What it actually means is this:

Proportionality: you use enough force for long enough to be sure that the threat is no longer able to harm you. That's it. You don't stop until the threat triangle (opportunity, capability, intent) is broken.

Doubt resides on the side of the defender - ie. if an adversary previously met the hostile intent triangle, and I'm not sure if they still have opportunity, capability, or intent, I continue to use force until I'm sure the triangle is broken.

Fin.