It's a domino effect: if he kills this 'young master,' someone older behind him will come forward to take his head. Then, after killing that old person, an even older one will come for him. The cycle will continue, with Apollo killing the next old person every time one of them falls.
Apollo wouldn't be surprised if he eventually had to face the first-ever human in this world. It's because of his audacity that people afflicted with the Young Master and I'm-old-so-I'm-right Syndromes exist in the first place.
Meanwhile, when Kuo-Pal saw the bright ball of light materializing in front of the beggar kid, he felt a force unlike anything he had ever experienced. He clearly witnessed it: Fire Blast and Wind Blast materializing separately before merging into one.
It was the same technique he had faced back on the training ground, but this time, it exuded an entirely different and unusual vibe.