"Ye have heard that it hath been said, Thou shalt love thy neighbour, and hate thine enemy.
But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;"
Those are the words of Christ himself. He's making it clear that, while under old testament law it was not only ok to kill in self defense, but even commanded by God at times. But, now under the Gospel, where Christ has come to condemn sin in the flesh, we have a higher calling to show that Divine Love is stronger than carnal hatred. Remember, it was Stephen's power to say " Lord, lay not this sin to their charge." that melted the heart of Saul, not his skill with the sword.
Also, consider that anyone raping my wife or trying to kill me or my family is obviously not a Christian and will be lost in Hell if killed in the act. How can I love my enemy and yet assist him in going to Hell? Had Stephen killed Saul in justified self defense then Saul would have never been converted and we all would have suffered from the loss of his writings.
I have no biblical authority to decide which life is more important, my child's or his murderer's.
Yes, it would be heart trending to witness that and not be able to stop it, but that is where " He giveth more Grace".
Now, I'll add this. If I can stop the violence without violence, ex: wrestling the perp to ground, tying him up, even sacrificing my life while the victim escapes." I will do so.
Take some time to study the life and teachings of Christ, get a King James Bible, and a dictionary if you have trouble understanding it, and then tell me which part of his teaching advocates defending yourself.