Some of the spirits obviously originate from humans. Now correct me on this, but why are the human spirits so powerful when in life the humans are so frail? Oh I am a human I am weak and frail, don't sacrifice me to Belial!..... I'M BACK AS A HAUNTING SPIRIT RAWR seems a bit weird. Why can't humans tap into that power when alive?
Also, if the spirit world is the source of the wd's power, wouldn't maltheal just kick her !@#? In the opening cinematic for RoS he just sucks all the soul like candy.