So? Those kinds of people are every bit as trashy as Nathanos is and nobody should ever have to experience being under such a person, but such people existing is hardly a problem compared to how poorly written Nathanos is. That'd be fine if people weren't used to better writing from Blizzard, but they quite rightly are, and he happens to be the most notable example of that downfall in quality in years.
You greatly overestimate that difficulty. Even the weakest of writers can take a character and evolve them in a direction of their choosing while maintaining the primary components of their personality - in fact, it's often a great way to get better at writing because you already know what limitations you're working with, unlike with a complete blank slate. That alone would be enough to forgive Nathanos if there was literally anything besides his bewilderment towards Sylvanas choosing to burn Teldrassil that showed any sort of tangible evolution in his writing quality.