I'm glad you've stepped in with your deep insights in child development and psychology to help us understand the situation better.
Kids have severely limited decision-making tools in their brains compared to adults - it's why a 14yo is not allowed to drive, drink, to smoke, to make medical decisions on their own, it's why they aren't considered legally capable of consenting to sex with an adult no matter how much they appear to have "consented". The fact that they are severely limited in decision-making capacity is not in the slightest doubt. Spend a year in an 8th-grade classroom if you have the least uncertainty on this issue. If every 14yo in the country had full-time access to a handgun, murders would skyrocket.
Yes, this kid did something fukking horrible. Yes, he should face consequences. Yes, "most" children would not have done it, even most children in his situation. But no, that doesn't make him uniquely evil or irredeemable, and you're basing that hot take on literally nothing. There have been plenty of people who committed more heinous murders at an older age than him and still turned their lives around.
The only two nations in the entire world that allow the execution of a minor are the USA and Iran. One of those does it because it has a history of being controlled by delusional theocratic a$$holes who know nothing of the true God, and the other keeps trying to start wars with Iraq.