Violence is never justified. Self defense doesn't even need to be violent if necessary. One could run away or take an attacker down with minimal force. From your aforementioned scenario and past karma, it sounds as if you are looking to vent your aggresion in violent ways, probably fueled by your skills and confidence in martial arts. I am definitely not judging you Teep, I understand where you are coming from. My feelings for justice and protection of others is also strong. But if we go out of the house expecting trouble or supposing that someone will give us a hard time, then this will be a self-fulfilling prophecy. This is just my own objective point of view. Only in the most extreme cases with no other options and as a last resort to defend a loved one's life would some form of violence be "justified" as an exception to this general rule. But I would not focus on exceptions, focus on the universal dharma of complete non-violence instead. Take care, and seek to channel your physical energies in positive and constructive ways through work and play.