Most medieval kings died of old age, illness, or some other "natural cause." If a king died from something more nefarious, it usually stands out in the historical record. Take the English monarchs, of which there have been about 50 if we count liberally between Alfred the Great and Charles I (by liberally I mean including people like Lady Jane Grey and Matilda).

Three were killed in battle or by wounds sustained in battle (Harold Godwinson, Richard the Lion Heart, and Richard III)

One king (Edmund I) died in a brawl that he probably started.

Three were definitely murdered (Edward the Martyr, Edward II, and Richard II).

Two were probably murdered (William Rufus and Henry VI [who was already deposed])

And two were beheaded (Lady Jane Grey and Charles I).

So, that's 11 deaths total that weren't natural causes, out of 50 people, and two of those are only suspicious deaths, not confirmed assassinations.

Edited to remove typos.

Your question is far too broad for anyone to answer (at least to the standard expected). I'm sorry. To explain: 1) the death of major figures is not always clear cut. 2) this means a respondant requires exhaustive contextual knowledge of a thousand years of history across medieval Europe (to assume you are just considering that region).

Our major sources at this level are chronicles - and these writers may be writing.some time after the events they describe. This might mean they have been influenced (or consciously decided to perpetuate) by rumours surrounding a monarch's death.

This question is terribly broad. Try and give it a more specific approach if you want an answer.