Long story short
Will was helping Lian (who is Medic) with experiments, when Will accidentally messed up, and caused an explosion, and a fire, which left Will as a bunch of burnt flesh, but still alive
Lian healed Will as much as he could, and to help with his new mental insanity, he made a special gas mask to make Will think that he was home with Lian, and all of the murdering, he thinks he's doing normal household things
Will went insane from the mental trauma, and gained an attraction to fire