You have brought up a very important point that is never taught in combat sport. The aspect of revenge. You may think that you won a fair one on one fight today but what about tomorrow and the day after, or even a few minutes after? If you do not kill your opponent on the streets, be prepared for a revenge, the person can come after you and take your life away. A person on the streets who cannot control his ego is likely to kill you if you hurt his ego. In the Boxing ring, the person that loses will ask for a rematch, on the streets, the person that loses will not ask for a rematch, he will attack you spontaneously when you are not prepared and he will kill you.
If you kill your opponent on the streets, you must also be prepared for his family members and friends coming after you to take your life. Real war on the streets is never glamorous, it is something we need to stay away from, but if we do engage we need to engage intelligently and always be prepared for the revenge.
If you let your opponent live, expect for him to come back to take your life, so be prepared, have your weapons ready to take his life away when he comes back to try to take your life away. Other things to consider as well is that your enemy may not even seek revenge against you, he may seek revenge against your family because they may be an easier target. Now you will start to see and understand why there is so much gang violence, gang members killing each other off out of revenge. We need to step away from all of this violence and head towards peace. We need to stay away from environments that attract negative behavior and violence. We need to train hard to be prepared for whatever happens.