Gaining invisibility is going to inherently be strong for any character regardless of class, race, or playstyle. Invisibility is often an investment of concentration or a rare and/or expensive potion, getting it regularly by landing a killing blow naturally lends a huge advantage.
Therefore I don't think firmly committing Durge to one or two classes or playstyles is warranted, at least not yet. After all, as others have already pointed out, we only know ONE reward/perk from giving in to their urges.
