So the King is promising rewards that the Kingdom can't quite afford.
[ul]The Second Prince is in a terrible position. To obtain succession, he's going to need at least the First Prince dead, and probably Kasdex and the King dead.
The First Prince benefits by having the King incapacitated, as it prevents the King from making more unsupportable promises. Once on the Throne in (or with the power of the Throne), he may be able to shorten the period of tax exemption.
Kasdex needs the King alive and on the throne to maintain the tax exemption while he consolidates his position. If the King dies suspiciously, and the First Prince is Dead, and Kasdex is prepared, he may pull off a successful secession from the kingdom.[/ul]