"Yaki-tachi wo saya ni osamete, masumasu masurao no kokoro wo togari keri." "Keep your tempered sword in its scabbard, first, polish a heart of courage."
According to Japanese culture, restraint and humility are the hallmarks of a true master of the marital arts. Those two traits are more important than strength, speed, ability or accomplishment. Anyone can cause harm or hurt other people, but only a true person of character can exercise restraint and practice humility in the face adversity. It takes courage to be a person of character.
In learning, one comes to understand that man is ignorant. That ignorance isn't stupidity but the lack of knowledge about humanity. A universal truth is that every person suffers and is going through their own stuff and thus sometimes lashes out. This lashing out is really them hurting themselves. This ignorance is what drives them - it drives us all. By studying a martial art, one realizes this idea of universal suffering. We come to understand that circuitously that it is not this person's fault and that they act in a harmful way because they are ignorant of their actions and are suffering. Knowing this we come to understand our own ignorance and realize that to destroy them will only hurts us. It takes courage to go against our fears and egos and demonstrate restraint to show compassion. In understanding their ignorance we are able to find the humility and strength to confront our own suffering and thus we are made better by this person's actions. This is the circle of life - we exist to help each another. Only with study can we come to not only understanding this, but to embrace it as well and thus we exercise restraint.
There is a scene from Akira Kurosawa's Sanjuro where Toshiro Mifune's character Sanjuro is trying to rescue Lady Mutsuta and her daughter who are being held hostage. He is angry over his inexperienced cohorts actions which alerted their captors to their escape. In trying to motivate his cohorts, Sanjuro suggests that he has to go and kill the henchmen because of their mistake. Lady Mutsuta over hears his chiding and says playfully, "You're too sharp. That's your trouble. You're like a drawn sword. Sharp, naked without a sheath. You cut well. But good swords are kept in their sheaths." Sanjuro acquiesces and instead offers to be used as a step stool so that they can escape out a window. She says, "Oh no I cannot, it would be improper." Sanjuro says, "Hurry before I have to go and kill more people" to which Lady Mutsuta gives in and they all flee to freedom.
Lady Mutsuta's high manners and demeanor made Sanjuro have to be a better martial artist. He was forced to become more like her and had to think and find a way to escape without killing. In that one moment with that one exchange, Lady Mutsuta made him not only a better martial artist but a better person too.