On Normal mode it does not matter much what build you choose, you could probably dip into all classes once and still succeed. Shadowheart in my playthroughs always went to Tempest domain after she turned away from Shar, it's a very strong build without any dip. If you want an easy time, it's better to have a Storm Sorcerer or so as separate char, for the obvious synergies of the (insane) condition the build cries to (ab)use. So maybe you could go Storm Sorcerer/Warlock, seems me dark enough a theme. However you had to accept that you could not get an important Warlock item (except you want to metagame or use a spawn mod).
Necromancer or Spore Druid for me would be too refined for a Dark Urge, too indirect and slow. Blasting creatures with Eldritch Blast or lightning spells seems the more appropiate way for such a tormented character bristling with dark energy. And Warlocks are known for weak morality, so that fits to not resisting the dark path.
I have currently parked my Honour mode before Orin and the brain and started a new playthrough with a dark urge character and some combat mods (which btw makes Honour mode appear as quite easy, sigh). I wanted to go Bardlock, but changed to a more slash-slash-bloody-rip-the-guts-out-idea, a bit like Crimsonrider proposed. Seems perfect for me when I look at what I do to people. I want to follow the good path however, let's see.
