I’d much rather have the right outcome (= not executing an elementary-school-aged kid for being the kind of ignorant idiot kids that age often are) for the wrong reason than the wrong outcome for the right reason (in this case, enforcing a law uniformly regardless of the identity of the accused—does that even constitute a “right” reason if the law itself is unjust?)

That’s totally aside from the fact that they still need his mana and therefore can’t afford to kill him. Enforcing the law uniformly might lead to a bunch of other, completely innocent people being hurt because they’ve destroyed a resource. In the real world, that’s the kind of thing that tends to get sentences commuted.