Deathrow or not he is serving his time and this guy doesn’t have the authority to just off him.
Because yes he’s a prince but a collateral one.
And being a prince he should know given such a serious yet fickle spy charge being dropped in favor of service to the state for a NOBLE CHILD.
Sounds like and is a political maneuver of the state to control Harold. Plus Besides that he doesn’t know the full situation between him and the sumeragis and if Harold were truly evil would put them in danger.
You are comparing a situation of monarchy against modern standards, I guess?
I mean, as a Prince, even if he isn't really high in-line of succession, he actually has all the authority to question & even execute MC, because position-wise, MC is still lower than him (both as a noble, and as a convict).
Also, MC's charges of being a Spy/Traitor haven't been dropped.
They just changed his Death Row sentence to working as a Lab rat for dangerous experiments, which have high chance of mortality (MC had survived until now because of his strength and knows some aspects of Justus).
And it doesn't really matter if a convict is serving his time. A convict is always treated as one, even after they have served their term and are free, especially if it's a high profile case.
MC's current work is something that has been assigned to him by Justus, and it technically doesn't fall under his usual sentence (these are just extra stuff that guy makes MC do).
As for the charges, no one bother thinking about whether the charges are fickle or not, especially in a time when information isn't broadly available.
All that Prince probably knows is that MC was charged as a traitor and is currently working as Lab rat for someone as punishment.
And yeah, he doesn't know the situation/relation of MC and the Sumeragi's, but that doesn't mean he would trust MC even if he knew, because MC's public image is that of a traitor, and there is no evidence to prove he is not (Sumeragi's know some stuff, but not the whole truth, and they don't have evidence either for the stuff they know, so they can't say anything to support MC as their whole family could be treated as traitors and convicted).
Also, state doesn't really care much about MC, as they also believe that he is a culprit. The only one who has any idea of what MC is capable of is Justus, which is why he manipulated stuff in the background to get MC under his authority.