Honestly, they deserve to just be taken over by a foreign state at that point. They could have just contacted someone and suggested becoming a vassal state at that point to avoid bloodshed and wrongfully killing an innocent person. Instead they feel they have some right to be what they are? why?
You're getting the priority wrong
Becoming a vassal state is not as sweet as written on the paper: That means becoming lackey to the country that defeat you. And that was especially true in the victorian-era world
E.g: Do you know how much tax could raise in short period of time in Aldebide just because Faris said them to do so, and how it would impact commoners? Do you know how many uprising would be staged by unsatisfied aldebide nobles, only for faris to say "please wait our reinforcement, for now please deal with them with your own military force" and send nothing in the end? Do you know how many MORE spies faris could place in the aldebide government in order to manipulate the country management and system, so faris wrong doing in aldebide won't leave any trace left when auditing took place by any 3rd party?
All of this could happen in era that lacked modern communications method, and
the world would be none the wiser about the truth
But even if they don't want that, start bringing up fake charges against those who are playing dirty too. I am sure they could have figured out something to lie about and go after those who are supposedly doing their duty. Clearly there's SOME evidence of TREASON if they're working with a foreign power. Even if they feel might not be enough, make it bigger than it is and getting the word out there.
It's no longer about fake charges or evidence of treason. If the execution of the general to take place, then faris would be able to invade aldebide with so much less resistance. Aldebide would be too busy to defend the country, when faris razed anything related about their conspiracy to the ground in the midst of invasion. Once again,
the world would end up be none the wiser about the truth
I wouldn't blame you to disagree about those 2's decision but please know that all those above could happen if the problems at that time are not solved as swift as possible, thus the choice thay made is scarlet execution