Edit: Additionally, the beings who pre-judged the trial (Shojo and Roy's Dad) aren't the beings who have the right to judge the trial. If they'd really wanted to be sure, they should have called in those Celestial beings of Law to ask their opinion. Of course they didn't, because they couldn't be sure that said beings would give the convenient result.
Again, a point in favor of Shojo not really being legitimate authority.
Heck, Miko even punished him in the name of the Sapphire Guard and the ancestor that founded it, which, I don't think anyone can debate, Shojo is very very guilty of pissing all over.
IMC, corrupting a lawful and good process just for your own expedience doesn't fall into Good territory. I don't buy "everything I did was for my people" as any more honest than "Of course this is a celestial being! And this trial is totally legit! And also, I'm senile!" The man has a reputation of lying to achieve his own ends. His end may be to keep the Snarl imprisoned, or it may be to control the final closed gate so he could extort the entire world to keep the great evil at bay (the Order didn't succeed at their last mission, why would he think they would succeed at this one?)
Again, the man's a serial liar, who fibs at the drop of a hat for his own convenience. His words being untrustworthy, we're left to evaluate his actions -- which could certainly serve evil ends just as easily as they serve good ones.
I say, even if he is Good, Miko doesn't fall for this one action. It's dangerous, but alignment has three components: motive, action, and reaction. The motive was Lawful Good (to restore justice to the throne), the action was debatably Lawful Good (applying standards of justice to one who had abused their power), and the reaction could be Lawful Good as well (Instituting a new, legitimate ruler in the throne, making sure Shojo can't hurt anyone else, etc.).