To be an initiate of Humakt you must have the Death Rune. Whether you have the Truth Rune or not is up to you. So, yes, having a Humakt intitiate with Darkeness, Death & Mastery is perfectly OK.
S:KoH (2009) had a "godless" sample character, Orlmakt Braveheart the Vengeful Thane, who wasn't initiated to any god. That's also a possibility.