Just so I am clear on some things...... If someone is under attack, it is acceptable to fight back by whatever means necessary.

Depends on their opponent. The means need to match the level of threat.

You say "necessary " but in reality, our perception of necessity can overlap with personal whims.

If a kitty scratches your leg and you chop its head, it is cruelty. Because there wasn't real life threat. It is easy to judge a person in that case. We can all agree it is crazy.

It is much more complicated than that when it is two people, one strong and the other is weak.

But in principle, everyone should have the right to defend themselves. Hopefully they just don't abuse that right and forget about compassion. There are certain situations where you just have to let it go because the person attacking you is ridiculous. Just punch or few slaps and that's enough.