The guys in charge of internal security, i.e. intelligence, counterintelligence, assassinations etc. can't off this king on their own?
They still need to convince the other nobles when this retarded king offed his own sole supporters and is pissing all over their feudal contract?
Why is the royal bloodline even necessary?? Just throw anyone up on the throne and rename the kingdom
This is the kind of thing that feels like it should be common sense, if everyone would benefit from deposing the king then just hurry up and do it, I agree, but it also isn't really ever that simple.
When people have been living a certain way for countless generations it's difficult to convince them to completely abandon that way of life all of a sudden, especially when there's a real chance of failure.
Asking people to put their own futures, not to mention their families futures, in the hands of others is difficult enough when you're dealing with just one group, but when there are potentially dozens or more groups that you need to all be on the same page for things to work, it's not surprising that things don't always go smoothly, especially when those groups have a preconceived notion of how things "should" work, and need that reassurance to even begin the process at all.
On top of that, they might be able to get away with killing the king, but just killing the king doesn't guarantee the continued stability of a country. What point would there be to killing the king if the state that the nation is left in is worse than before.
tl;dr, you can't do politics by yourself, but having too many people involved always makes it more annoying than it needs to be