MASSIVE SPOILERS:
In an interview posted on the subreddit, Swen mentions that Dark Urge offers the most evil way to play and that you can become an evil god. With full divine powers.
When we combine this information with:
-The fact that we see Dark Urge in Bhaal's symbol in the trailer.
-We see Dark Urge become the Slayer
-We know the Slayer is either the form of Bhall himself or Bhallspawn
-We know Bhaal just wants to kill and bring forth bloodshed from living creatures
-We know Blood in Baldur's Gate occurs before Bhaal's return and that Bhaal is not a full god, but a quasi-divine being, whose murderer (Cyric) still has divinity...
It seems pretty clear to me that The Dark Urge is Bhall, an Avatar of Bhaal, or an Incarnation/Manifestation of Bhaal, or a descendant of a Bhaalspawn (The Kid?). Perhaps the Goblin-like creature is either Bhaal actually or his Imp Butler Cespenar
For me it's really if this is a red herring or not. Alternatively, if we're not the one experiencing the dark urges -- a boss we fight may be the person who did... because it's an actual canon character Larian isn't wasting. Essentially, it's a way to become that boss, as the complimentary dark side.
