Charlemagne had a younger brother, which was very inconvenient because by the rules of inheritance at the time they were required to split their father's kingdom.

Charlemagne was particularly put out. He had plans to conquer Europe &/etc, and an uncooperative little brother was going to be a drag.

Then Carloman mysteriously died, still a young man, in circumstances that were never explained, and no cause of death was ever given.

It was very convenient for Charlemagne though. He promptly conquered Europe and founded the Holy Roman Empire.

Which probably would never have happened if Carloman had hung around. Charlemagne was never actually accused of arranging the assassination of his little brother, however this must be tempered by the observation firstly that Charlemagne was incredibly good at planning things, and secondly that only a crazy person would accuse the Holy Roman Emperor of murder.

Do feel free to add your favourite assassinations in comments. Somehow I have a feeling people will have their own lists.