Arthas made the right call at Stratholme but only in hindsight. For all they knew at the time a cure was literally one quick trip to Dalaran and a short baking project with Antonidas and one of KTZ’s journals away. It could have literally taken half an hour to return with a one size fits all cure for all they knew. But Arthas jumped straight to murder, which understandably alarms his allies.
"For all they knew", mate, they were standing literally outside a city with citizens turning into zombies. Let's take a stroll back to Dalaran and let them multiply and hope we get a cure. If we don't, fantastic - we let a group of undead run rampage, who knows where they may be by now!