When you think about it, there was enough complex decisions taking place to point towards a developed (albeit devoid of conscience) mind in control.

A childs reasoning would more likely follow this kind of path: Child wants alcohol->storekeeper has alcohol->child capable of coercion by force (ie stabbing)—>child gets alcohol from storekeeper

However, instead he did this: Young man wants alcohol->storekeeper has alcohol—>alcohol costs money that young man does not have—>innocent passerby’s have money—> young man capable of coercion by force (ie stabbing)—>young man stabs innocent passerby’s for their money—>Evaluate whether amount stolen is enough for alcohol, if not, repeat previous steps, if so—->young man gets alcohol from storekeeper

Good decisions are not proof of brain development, however complex ones are. Given the evidence I would say he was rightly judged as an adult.