Like I said, it is unjust to treat a 16 year-old as an adult in such a circumstance while not treating him as an adult in others.
If a 16 year-old is mature enough to be executed for his crimes, then he is mature enough to vote or enter into legally-binding contracts.
A 16 year old who murders is acting like an adult and is a threat to society thats why.
And for the record 18 year olds can be forced to serve in the military and cant drink alcohol so what is just based on your logic is red herring.
If 16 year olds can hold life-and-death in their hands in the form of a motor vehicle, can earn a paycheck, can pay income taxes, can (some states) legally have sexual relations, can concent to abortions, then a 16 year-old can face a death sentence for murdering their peers, their parents, etc.
A 17 year old is hardly a hair's-breadth difference from an 18 year old.