Dhamma123 said: ” I live in a world where violence in speech/body can be useful for self defense.”
It is up to each person to decide how to live one’s life. The Buddha only pointed out the consequences of violent acts against others. It does not matter whether one is a bhikkhu or not.
You said: “Lord Buddha said it is impossible to live in this world without harming anybody..”.
That did not include violence. That meant, for example, in following cases:
When one walks on grass, one may be killing a lot of small insects. When one applies medicine to a wound, one may be killing a lot of microscopic living beings. Those are not intentional and cannot be avoided.
But hitting someone, just because that person hit you, can be avoided. And it will save a lot of suffering in the future for oneself (and for the other person too, since if you hit him he may hit back again). By the way, this is what is really meant by being mindful (Anapana/Satipatthana).
Violence CANNOT be stopped by more violence. That is what y not explained above.