Most people would say that Arthas' turning point was the Culling of Stratholme. I disagree. I think that it was a difficult decision, possibly made easier by a desire for vengeance, but rational nonetheless. I realise that others may disagree with me about Stratholme, but bear with me and argue in the alternative. If not the Culling, then when exactly did Arthas cross the line? I think taking up Frostmourne is a clear turning point but from that point he could be considered under the influence of the Lich King and so I do not think it is fair to completely attribute his actions to him.
My next best guess is when he burnt his ships and back-stabbed the mercenaries and soldiers in Northrend. If this is the point in time, then why exactly? I don't think that a desire for vengeance is well supported and doesn't fit well with his character. I find it hard to believe a prince who is also a paladin would be overcome by vengeance so easily. I would believe it if he was influence by, say, an Old God or Sha.
So, when exactly do you think Arthas cross the line (if not Stratholme or taking Frostmourne) and do you think he was under some extra influence at that point?