Earlier quoted context omitted.
I think you're seriously underestimating the rate of violence against kings. They didn't live in ruinously expensive, heavily fortified castles for nothing. They endured constant plots against them, from everyone, and to misjudge those threats meant death.
That would be an interesting study, actually, to see what percentage of medieval kings died of old age (and not in their successor's dungeon). As a first order approximation, I've looked at the Wikipedia article "List of monarchs of the British Isles by cause of death"[0] (which covers more than just the medieval period), and counted this summary: Natural causes: 60 In battle: 15 Murdered, assassinated, executed: 17…
There's also the wannabes (like princes) who were killed to prevent their ascension to the throne.