About the descriptions on the Wiki, we just base these descriptions on the hero's abilities and regular attack (as well as any comments by the Edain Team).
So for Khamul, his abilities let him passively damage nearby buildings, disable an enemy building and summon his Black Easterlings who are strong against buildings. We've therefore classed him as building destroyer. Saruman has wizard blast, fireball, lightning and an AOE regular attack, so we've classed him as mass slayer. Both of these heroes have other skills that make them useful in other ways (for example Khamul's morgul blade and debuff or Saruman's convert allegiance and Power of Speech). However, we just try and focus on a main role for them which is entirely dependant on how the player uses them.
Hopefully that helped.
And just out of curiosity, what would Elrond's description look like? "Hero supporter/unit supporter/tank/mass slayer"?^^
Yes there are some heroes who just have a large amount of roles. These include Sauron, Galadriel, Faramir and Haldir. With these types of heroes, we've either just listed their roles or put them in a new category of All-Rounder.