Should they have tried quarantining them until a cure could be found, perhaps? (Even though there isn't one.) It's a bit like a recent Fringe episode. Was it evil to consider killing all of the people infected with an extremely intelligent, contagious and fast-spreading disease? How do you deal with deciding between compassion for a few versus the survival of a race?
Could the ruthlessness that Arthas showed there be a symptom of weak morals that perhaps led to his demise as a human? Or was his swift, decisive action an example of his excellent leadership qualities and why he makes such a successful Lich King? Perhaps doing the right thing in Stratholme weakened his soul, making him more susceptible to corruption.
How should Arthas have behaved in Stratholme? Did his actions help corrupt him or show him to be already corrupted? What would you have done in the same situation?