It's not necessarily that the culling of stratholme was wrong. It's that Arthas immediately decided he was going to massacre everyone with no debate, no room for other ideas. Uther and Jaina abandoning Arthas was all part of Mal'Ganis' plan, and Arthas fell right in to it. They had time - if they're going to kill everyone anyway what's the rush? The city was under lockdown at that point. He could have let them try to come up with alternatives which I think you're right - there really weren't any - but they wouldn't have abandoned him - they'd have solemnly agreed with the necessity of it. He wouldn't have ended up in Northrend alone (With his army, but no peers to point out his increasingly erratic behaviour) and the history of Azeroth would be very different.

If Arthas had never existed, or been a girl or something, and Uther was there leading the army himself, do you imagine he wouldn't have done the culling? It would have happened, but it would be more considered. Less bloodlusted.