What did the Lich King do that made him so evil

It was not a single event that created the Lich King. Arthas was intolerant for a Paladin, and it was his decision to cleanse the Scourge-stricken city of Stratholme by force rather than healing that set him on his path to evil.

In his mad quest to avenge himself on Mal'ganis, Arthas travelled to Northrend, where he betrayed his men by marooning them there. This he did to prevent them from returning home instead of helping him.

Finally, when he drew Frostmourne from its altar in a cave in Northrend, his evil was complete. He killed his father, and went deep into Northrend to sit on the Frozen Throne and join with Ner'zhul .

The Lich King was born.