I just watched this film and really liked it. My only problem is, I can't quite figure out Martin....

Throughout the majority of the film, I thought it was made pretty clear that Martin was a conniving psychopath who was pretending to be intellectually disabled in order to get closer to Susan.

However, the ending kind of changed all of this and suddenly implied that Martin had dissociative identity disorder, with "Georgie" actually being an alternate, childlike personality he'd regress to.