@TNT261 I can't agree more
Sending him to the island is way more costly than having him killed right away, they even gave him supplies not to mention the transport fees.
But in this situation, I'd like to think that the reason why he's not executed at the spot, is because of political reasons. killing a royalty will give a bad image to the subjects, and citizens.
Sending him away from the country will at least reduce the blame directed to the palace and instead the palace will blame the MC death is because of his incompetent.
If he's that worthless, he still has the royal blood running through his vein. That itself has a high value for political agenda such as political marriage and diplomacy.
But in the end, it's just a fiction story, there's no point trying to push logical reasons to a fantasy story.