All started because Arthas made the choices to go down his own path to corruption.
You act like slaughtering Stratholm was the correct answer because you, personally, don’t see other options. Jaina and Uther who both were present, didn’t think slaughtering the city was the right call. What else could have been done is moot, because without trying to find another solution, Arthas jumped right on the slaughter wagon.
Could the city have been contained with the huge gates we see? Maybe, maybe not.
Could they have separated the infected from the non-infected? Maybe, maybe not.
Did killing everyone in Stratholm kill innocents? Yes. Did it stop or even slow the spread of the plague? No, not really.
Was it the right choice? No.
But was it the moment Arthas showed himself as a corrupted person? Before losing his sanity, before picking up the sword, before even going to Northrend?
Yes. Which was how this whole debacle of a debate comparing Sylvanas to Arthas began. Pre-banshee Sylvanas clearly was, if not a good person, at least a moral one who died protecting her people. Pre-Frostmourne Arthas slaughtered a city because he didn’t want to listen to smarter, wiser voices.
Frostmourne didn’t destroy Arthas and make him into an evil thing; Arthas was an evil thing and the Helm removed the bits of him that weren’t evil. We actually saw this in Wrath (go back, do the Icecrown zone quests). His own actions prior to the sword, prior to the helm, showcase that at best, they removed his inhibitions.
Interestingly, lost in all of this is that when Sylvanas was killed, her soul was transformed by Frostmourne (warbringer cinematic). If you believe Frostmourne utterly changed Arthas because he weilded it, and all Arthas’s actions from then on were controlled by the sword, then Sylvanas gets the same excuse for the exact same reason. The sword made her into what she became.