Bro committed treason multiple times, tbh any competent king would have executed allan even if only so that people don't think they can get away with lying to the king like that. like that's a capital offense in a monarchy
The problem is not the king but the author.
Those kinds of actions would have either resulted in capital punishment or at least lifelong imprisonment or forced labor in any kind of monarchy.
Especially in Japan where honor and honesty were kept in extremely high regard.
Even if you do not consider Japanese historical values the fact alone that a person with doubtful morals and a dangerous power (be it a skill, ability, magic or weapon) has commited criminal offenses would make it impossible to let that person walk around freely afterwards.
Yet Alan only gets a "criminal mark" and can roam around freely?
That would not happen in any society ever.
So this is an issue of the author himself not having any functioning logic.