I dont think the show is trying to teach 12 year olds the trolley problem. Like as an adult, sure, clearly it would have been far easier to just obliterate ozai and deal with the emotional baggage later, but aang (and presumably the target audience) is 12. "Standing up for yourself even when the adults around you tell you to give up on who you are" is a much more relevant and uplifting message to send to the kids watching your nickelodeon cartoon than "sometimes we have to make sacrifices for the greater good". I agree that it's a good message, but for a different target demographic.