Why Was the Villain a Rich Kid?
It had no bearing on the story, and he would have had more pathos if he'd been raised in poor surroundings and worked his way out of them.
Dr Sivana and Lex Luthor are self-made men in the comics. It adds to the complexity of the characters, being these working-class guys who had to work hard to raise themselves out of poverty. By making them privileged rich kids, as the recent DC films have done, it undermines their sense of grievance and simply makes them two-dimensional entitled jackasses.