[jfengel]

Medieval European nobility tend not to die in battle. They were captured and ransomed. Richard III died in battle but nobody was gonna ransom him.

Assassination definitely brings down the average. But a fair number of English monarchs managed to die in bed. (I was gonna write British, but no: the Scottish kings practically never died in bed. Unless they were stabbed in their sleep.)

[tsimionescu]

Fair enough related to battle, but that's not the only risk of waging war. A fair few kings caught diseases from their campaigns - which they may not have at home. Overall from some light Wikipedia browsing, it seemed to me that around 1/3 of medieval English kings died from assassination or battlefield wounds & diseases. Note that Richard III is basically the last Medieval king, I'm talking about earlier periods.