As others have said you're totally right about the guy who got hit by the ball. He should have gone around, and especially since it bounced off the hoop it was not your fault at all. He has psychological problems and deserved to be told off. People like that are absolute scum, and he had absolutely zero right to see your ID or try to make you go anywhere with him. If you had deliberately drilled him in the head with the ball then yeah he would be right to call the cops but he still has no right to see your ID or try to bring you anywhere. However this was entitrely his fault, no different than if he had walked in the rain and gotten wet. We call this kind of person 꼰대 (kkondae, an stubborn, entitled, combative elder).

As for the kid... Yeah he should have gone to the other net but it's just a kid who wanted to play with a foreigner. Kids (and adults) do stuff like this. With an adult of course they should know better but in either case it probably wouldn't be ill-intentioned. If it happens again and it's an adult you could try to politely explain but the better option, especially if it's a kid or an old person, would be to say hi and share for a bit first then move to the empty hoop and, if possible, explain why you did so. Korean culture is big on sharing and courtesy.

TLDR: The old guy who got hit with the ball was a psycho by any cultural standards (good for you for standing up to him). The kid was just trying to make friends and, while perhaps a bit obtuse, was likely well-intentioned and should be understood.