Can't say for all monarchs through human history (nor is it even possible to), but for the Roman emperors, 70% died violent deaths via assassination (actual and suspected), execution, suicide, or being KIA.

You had some particularly colorful episodes like the year of the four emperors where Nero's regional governors revolted and he committed suicide. Galba, a general, was declared imperator by his troops, only to be overthrown and killed by the Praetorians 6 months later.

Otho seized power, but died by suicide after he was defeated by Vitellius 3 months later.

Vitellius was declared emperor by his troops after defeating Otho at Bedriacum; held onto power for 7-8 months before being defeated & executed by Vespasian at the 2nd battle of Bedriacum.

Vespasian seized power and had himself proclaimed emperor by his legions, reigning for ~10 years.