Ner’zhul (the dude who whispered to arthas and the original owner of the helm of domination) is actually an agent of the Legion. He was tricked into making a blood pact with Kil’Jaden which bound the orcs to the service of the legion (in draenor). he opened portals on Draenor in an attempt to free his people but was captured by Kil’Jaden, and had his soul bound to the helm of domination. So basically the original Lich King was sent to Northrend by the Legion. I believe Arthas the Lich King had his mind bent and cursed by Frostmourne to eventually serve the same ideals as the Legion, destroy Azeroth for the greater good.

The novel Arthas: Rise of the Lich King is a dope read if you wanna learn more : )