Well...

Like most humans, Charlemagne was a complex person. He was capable of being a good ruler and implemented various reforms to education and writing, and he promoted art, literature, and scholarship. He was also passionate about bettering himself and spent a great deal of effort in his later reign on becoming more learned. Whether or not he actually ever learned how to write is in question, but he certainly tried and we know he personally studied under tutors of grammar, logic, astronomy, and mathematics. The man was a law-giver who formally codified the Frankish legal system and sought an official, structural judicial system. He was also kind to the Jews, employing them in his court and as ambassadors, as well as inviting Jews to settle within his empire. Overall, under his rule Western Europe experienced the "Carolingian Renaissance," a period of cultural flowering and revival.

On the other hand, he was capable of extreme violence. At his infamous "Blood Court of Verdun," Charlemagne massacred 4,500 captive Saxon pagans, who had rebelled against his conquest and subsequent christianization attempts.