He has a really distinct character arc that occurs gradually over the course of the campaign. But, when his people are threatened by a new enemy, the undead Scourge, Arthas is pushed to make harder and harder choices that lead him down a dark path. He's the golden boy, trained to do good and destined to be great. The Human campaign focuses on Prince Arthas, who is a paladin and heir to the throne of Lordaeron. The main one being that it's a really great character story. This is the story of Arthas' fall from grace.

Especially the first Human campaign, The Scourge of Lordaeron.

So I recently replayed WarCraft 3 just for kicks, and damn, the campaign is excellent.