I took a gander at the word translated "obsessed" or "mentally Ill", and it seems either can be used. http://biblehub.com/greek/3552.htm As to WHY the WTBTS decided to change it, I would assume that they want to drop the belief that a person who goes against their teachings is literally insane. After all, a person who is clinically insane cannot be honestly held accountable for their actions, while an obsessed person absolutely can.