He murdered his beautiful royal wife, the Princess Mariamme and the 2 handsome sons he had with her, strangling them with a silken cord. Despite his cruely and viciousness as a ruler, he also kept the 1st century Israel out of trouble with the Romans by collaborating with Roman officials, something very few people could have done and built cities, palaces and fortresses whose ruins still impress.
King Herod the Great is the main antagonist of the Christmas story.
He ordered the slaughter of the innocents in Bethlehem, at the time of Jesus's birth, as Matthew narrates? There is no other evidence for this event, but it would have been quite in character for King Herod to do something like this. He saw plots against him everywhere and given the number of people he put to death, there were probably a fair few plots for him to fear.
Certainly he was hated by a great many people, after he had failed to kill Jesus by eluding him as the Holy family escapes to Egypt, King Herod orders every boy under the age of 2 killed, according to Matthew, it was the massacre of the innocents, because King Herod was first looking for a man, then he realized it was a child with a special gift he was after. His greatest achievement apart from switching sides at the right moment from Mark Antony to Augustus was the rebuilding of the Temple in Jerusalem.
This mammoth task began in 19 BC and took many decades to complete Jesus, Mary, and Joseph saw a work in progress when they visited the Temple and it is even possible that Joseph may have worked on this or one of King Herod's other mammoth building projects. King Herod was disliked by the Jewish population he ruled, due to despite his title "King of the Jews" he raised Pagan temples, ignored Jewish law and had Rabbis who disagreed with him put to death. This and the ambiguity of his lineage led them to speculate he wasn't really Jewish. As King Herod lay dying in terrible agony, he ordered that as soon his soldiers were to execute several hundred popular officials, so that there would be a public lamentation throughout the city at the moment of his death.
Fortunately, his sister countermanded the order but the incident gives some idea of just how his crazed mind worked.