Roy may not know it's Tarquin, but he has good reason to suspect it is: the unknown fighter is much, much more powerful than the "leader" Nale, yet does not wish his identity known. That's Tarquin's standard shtick, the power behind the throne.
As for thinning the herd, I predict a whole unveiling of betrayals on the side of Team Evil.
SpoilerTarquin probably won't last past the end of this Gate. If the Star Wars metaphor holds true, he'll save Elan by throwing somebody (probably Xykon) into a shaft, a la Vader and the Emperor.
Zz'drti will realize Qarr fed him bad coordinates for the Plane of Ranch Dressing.
Malack may take the opportunity to attack Nale.
Nale may wonder if Sabine abandoned him; he didn't hear her say "I love you ... forever" nor his father's explanation of her banishment.
If Xykon dies (as above) and must regenerate again, he'll discover that Redcloak switched the phylactery.
There are, in short, a great many betrayals on the boil.