Thinking about this purge of Dalaran... and the purge of Stratholme....
You have:
1. A leader that feels betrayed time and again by the horde.
2. A leader that feels he must protect those who are not infected (other cities) by an incurable plague.
I never felt that purging Stratholme was a bad deed. What is better? To let every person transform into the undead and feed upon those who have not yet transformed? Or do what Arthas did? There wasn't much of a choice imo.
Ofcourse we have the Runeblade Frostmourne to blame for the change in Arthas. But it started with this....