why didnt they know the killer would survive????

Rating: Null/10 (6 April 2004)

i have been reading most of the posts about this movie, and i agree with what most of the posts have to say...but i have one question...
the doctors were trying to "kill off" the personalities right? what made them think that that "killer" personality wouldnt be the one doing the killing? even if they were just trying to "get rid" of the personalities...what made them think the killer personality wouldnt be the one doing the getting rid of???
im not sure if that makes complete sense, and if someone has asked that already, i apologize, but the question jumped into my head while reading all of your posts...