The Ten Commandments have mainly been important in two ways in European history. They have served as moral guidelines (though they have clearly not prevented people from acting in horrible ways). They have also helped to cause people to put more importance on the Church, particularly before the Reformation.
The Ten Commandments have never, sadly, actually prevented atrocities from happening in Europe. People who professed to follow the commandments were perfectly happy to enslave others. They were happy to start wars like the Hundred Years’ War. They were happy to let the lower classes have wretched lives while they themselves lived in luxury. There was plenty of killing, stealing, lying, adultery, coveting, and other violations of the Ten Commandments.
However, that does not mean the Commandments were irrelevant. They did serve as a set of guidelines to which people could and did aspire. They provided a vision of how people ought to live.
Perhaps more importantly from a tangible point of view, the Ten Commandments helped to put the Church in a position of great power. People came to feel that the Church was the only entity that could save them from the damnation that they deserved for breaking the Commandments. This led to a situation in which the Church had a great deal of political and economic power.
Thus, the Commandments have had an impact on European history even though they were never followed very well by the people.