No, of course not. That royal on a diplomatic mission savagely arrested. 
Regardless, though, there was a specific treaty against executions of prisoners at the time. Breaking the treaty was supported by a great number of Riombara's nobles, and the offender was merely asked to leave, where he then went to serve in an allied realm that later backstabbed Enweil. Southern propaganda might have made people forget, but *that* is the reason that Enweil was on a war for the destruction of Riombara, lest it receive the head of either the executioner or Delvin as compensation, which (unsurprisingly) Riombara was never willing to grant. Executions of royals should not be taken lightly.
A realm willing to recognised the "Royality" of someone captured as a infiltrator should also not be taken lightly, no matter the excuses they try to come up with to mask the game generated message that they were captured in the act. My understanding was your "royal" had used their status to avoid such a fate before, guess things just caught up with them.
