@_Fallen_King_ @Elhao : the why is normally a combination of a few things :
[ul]
[*]They are a threat the the rulers power and/or influence.
[*]The rules is just a jerk/evil
[*]They ruler want to take credit for their efforts
[*]"Revenge" for some (imagined or accidental) slight (possibly the prince's missing arm)
[/ul]
We will see what it is, but usually those are factored in there somewhere.
(OH and sometimes the slight is not being a noble of that kingdom)