Arthas was always spoiled and insolent.
He was never truly "possessed" by frostmourne any more than it allowed him to follow his darkest ambitions. In essence, never, in the course of becoming the Lich king, was he forced to do anything he didn't want to. So was he truly "corrupted?"
Arthas wanted to be king, yes? A great leader? He wanted revenge on Kel'thuzad. He got it; kel'thuzad became his doting servant. He wanted revenge on Mal'ganis, yes? He got it. Ran him through with frostmourne. He wanted to be a king. He got to be that, too, and with a kingdom far exceeding the bounds of Lordaeron. He commanded a force far more powerful than the silver hand ever was, and Uther, who ridiculed and rebuked him? Slain by his own hand.