[@Inkd Bull] I should explain better. 

In my mind, the guardian was once a healer. He was given a charge to look after, a pretty at-risk kid. You know, troubled youth or whatever. However, the guardian was young (as spirits go) and irresponsible. As a result of this irresponsibility, the human dies. Maybe they were killed, or died in a preventable way, but it wasn't anything natural. I would imagine this is a horrible "sin" in the spirit world, and he was punished, stripped of his healing power and cursed to darkness and decay. Like you said, maybe cast out with the evil spirits. 

Then, a long time later (AKA now) somebody somewhere decides to give him a chance to redeem himself, and he is assigned another charge. He is now jaded and changed from his time in exile, to the point where he is no longer recognizable, physically or emotionally.  That make more sense?