Arthas was a classic tragic hero who was faced with two horrible choices and by picking one he lost his humanity. The choice was what to do about towns and cities where people were being turned into monsters by a plague.
His choices were
- Try to save as many as he could, which was a long shot at best or
- Save the town by killing everyone. He chose the latter.
That is the kind of situation you see in a Shakespearean tragedy and it is what makes Arthas a great character.