Were there any murders of rulers or people in power that were well known during the middle ages? What was the reaction to these assassinations? If not, was there any particular reason for no medieval ruler to be assassinated? Or is the act of assassination more of a modern concept that doesn't fully fit with the medieval world?
By definition, an assassination is a murder of a notable or political person, not just a ruler. They often take place when there is a disagreement about who has what power, as in the first that leapt to my mind, Archbishop Thomas à Becket, 1170.
In the Crusader kingdoms, it was considered rude to call oneself the king of Jerusalem, Christ's city. But Conrad of Montferrat filled that function until an actual assassin got him in 1192.
King Przemysł of Poland was murdered in 1296. The 13th C was big on assassinations: 1286 King Erik V Klipping of Denmark; HRE Pilipp von Hohenstaufen; 1225 Archbishop Englebert I von Köln; 1233 inquisitor Konrad von Marburg; and so on.
Lots of so on.
So, yes, assassination was as much a tool of politics and personal vengeance as ever, down the whole thousand years of the European Middle Ages, from numerous people earning sainthood that way, and Roman Emperors like Julius Nepos (480) down to the Rennaissance (1419, Jehan the Fearless, Duke of Burgundy).
There were so many one can't describe the reactions except on a case by case basis. In some cases, like that of Étienne Marcel, provost of the merchants of Paris, 1358, it was the mob that killed him. In others, like the inquisitor, most people would buy the killer a drink. One has to look into the particular politics of each situation, starting with how upset the (new) ruler was, which might be extremely or only pro forma, or if this would start or end a war, or if the Pope excommunicated your area over it.