It basically boils down to, "Hey, Arthas is doing stuff. Eeeeevvviiillll stuff."

The third chapter of the ongoing story behind Wrath of the Lich King is up at the official World of Warcraft site.

All of Lordaeron rejoiced at Prince Arthas' triumphant return from Northrend, but their elation was not to last. Not long after Arthas knelt before King Terenas, the former paladin thrust Frostmourne into his father's heart. At the Lich King's command, the fallen prince went on to murder his mentor, Uther the Lightbringer, and claim Kel'Thuzad's remains. When Arthas had finished, Capital City, formerly one of humanity's crown jewels, had been reduced to a silent wasteland of death and despair.

You can read all of it at the official site.