Umm...the reason he didn't kill Arthas, was, and I'm not saying it's the full reason, but Arthas, technically isn't dead...He is still a living breathing human, that has immense power, kind of like Illidan. He's still a Night Elf with Demonic Powers and features. Ashbringer, COULD kill Arthas, but remember, Arthas was the king of Lordaeron. This is a theory, but in the medieval times, if a King dies the prince would become king. Well, Prince Arthas, killed his father, and became respected king of Lordaeron. And his people knew how powerful he was, so if they tried to kill him, they'd be screwed over. Mograine was all the way in Southfall around the time of Arthas betrayal. After creating Ashbringer, he went up to EPL and WPL and was killing the Undead that Arthas didn't kill when he was on his way to Stratholme. I'm sure he was pretty busy with all of the Undead their too...I mean, it is currently, by means of technicality, the Scourge capital on Azeroth. Atleast until Northrend opens up.