Self defense or defense of another person's life where the situation cannot be deescalated non-/ less violetly and calling the police is not an option.

The force used should also be proportional to the situation. A grown man doesn't have to sucker punch an unarmed 100 lb woman with full force to defend himself from her, though in the reverse situation the woman would probably need to use all her strength just to get away from him (chances are she can't knock him out, let alone hold him down for even a few seconds, no matter how much she tries).

In an actual (or percieved) life or death situation it is understandable that one might panic and just punch with all the adrenaline they have though - if there's a 100lb methhead chasing you with a knife, it's still terrifying.

That's slightly more liberal towards self defense compared how the laws work where I live but that's my line.

Generally violence should be avoided but in situations where this is not possible without injury to yourself or other defenseless people around you, the amount of force needed to deter, restrain, or otherwise stop them is what is appropriate (and it heavily depends on the situation)