He's always been shown to be a more noble person overall and not to mention he's already been hinted to be a big Holy person. The focus is just on other characters(Anduin is not really used as much but I'm sure he'll pop up in 8.3(If its the dethroning Sylvanas part of the story). I don't think him suddenly using a shadow ability(Thats in a video game and don't take everything so literal) means he will turn bad or being disc.
And for the record he actually has developed, he isn't the pacifist that he used to be. THe fact he put himself on frontlines and fight against the Horde is a pretty big difference.