However the teenage brain develops, a 15 year old will have a clear idea of what is considered right and wrong in their society - and killing people is firmly on the wrong side of the boundary.
Where the clueless teenage mind comes into play, is their deluded belief that they would never be found out, despite leaving clues everywhere.
Nor had they thought forward about about what the punishment would be if they were caught.. They are now in the prison system for the next 20 years at least. When their contemporaries reach 18 and are bursting on the world, going off to university, starting jobs, meeting up with friends, going on holiday, they will be prisoners banged up in adult prisons. Never going out, never smelling the breeze, seeing spring flowers, watching lambs gambolling, swimming in the sea.
When their contemporaries are partnering, marrying, having children and careers. They will still be banged up in adult prisons.