@Doomer To be fair, I doubt executing him in this case would have done much. He is a warrior, and a threat of death is likely not that much of a matter to him. While his death might relieve some of the resentment and anger of the populace, it is his being alive that would benefit the kingdom more. Kinda like a necessary evil I suppose, at least at this point in time.
But I agree that he does need some suitable punishment. I don’t know if his current situation can be counted as one. That remains to be seen. He didn’t get off Scot-free though, as his beloved raised mercs are pretty much taken hostages by the emperor at this point (at least that is what I think the emperor implied). He would always be kept on a tight leash in the foreseeable future. He would lose his freedom to act, which seems important to him.
One way I could think that would make him squirm though is to execute his raised mercs one by one, every last one of them, in front of his eyes. I was about to add that they could also be told that their death is on their leader’s stupid head to break them beforehand, but it could also backfire.