And how is betting of people hitting each other being considered as legal and ethical in today´s world?
It's legal because there are precautionary measures being taken to ensure the safety of the competitors, they are not fighting to the death, once an opponent is clearly seen hurt the fight will be stopped, and both competitors are made sure they are on their peak of health, so it's really a battle of speed and strength not savagery.
Not only that there are many measures that are being taken to protect the fighters, even if the fighter wants to keep boxing the referee can stop the fight, their coaches can stop the fight and the doctor can stop the fight too, so despite how brutal the sport may seem to many people there are many precautions that are being taken in order to ensure that the fighters are in top shape and are healthy enough to engage in such a demanding sport.