Maybe because he's literally Satan, the obviously evil-looking guy who rules over Hell and wants to rebel against the Heavens?
Like is this so hard to understand?
Why do you think Sargeras needed to hide his true appearance by looking angelic? Why do you think Kil'jaeden needed to hide his true appearance by taking the form of Ner'zhul's wife? It's not rocket science.
Yeah, a lot of Horde races look like monsters, and anyone who hasn't played WC3 will tell you that the Horde is the evil faction and the Alliance is the good faction.
Why do you even want to defend Sylvanas lol, you literally agreed with me that she trusted a Dreadlord, making her an idiot.