I readily concede the fact that a 15-year-old's frontal cortex is not completely developed.
I reject the idea that only a fully mature frontal cortex is capable of restraining someone from murdering a teenager. Even a radically undeveloped frontal cortex is more than capable.
This kid went out that day with a deadly weapon, seeking out the person or people who had previously attacked his friend, intending to commit violence against that individual. He found this teenager. Based on this teen's race, he believed this teen was complicit in his friend's attack. He spent 4 minutes arguing with this teenager, then stabbed him.
This wasn't a crime of passion. This was premeditated. He left his home that day intending to use his knife on someone. He knew his actions and intent were criminal and immoral, and he chose to act anyway.
Everything else in your last comment is an ad hominem, and doesn't need a response.
This is another ad hominem, disguised as an appeal to authority.
Correct. You were describing me, rather than discussing the issue. That is, by definition. An "argumentum ad hominem". It is an "argument against the man" rather than an argument regarding the issue under discussion.