Its funny how they threw in a Light-harnessing Nathrezim, just to give the Illidan redemption thing more substance. Its clear which point they've tried to make; if a Nathrezim can be redeemed, then so can Illidan, so they just threw in a member of the most unreliable and devious species we've ever encountered and made him a champion of the Light.
A dreadlord with shiny armor comes in a spaceship, helps the rest kills one of his own(nothing new and nothing spectacular; Varimathras helped Sylvanas kill his own brethen too, while still serving Sargeras' agenda) and everybody is completely comfortable about it. Lothraxion could be a direct servant of Sargeras, just like Varimathras was. Killing or helping kill a dreadlord doesn't necessarily make a dreadlord good, as was witnessed in the past.