By the time Mort has his breakdown, it's clear that the other "Mort" trying to reason with him is his conscience and that Shooter is the part of him that wants to kill Amy and Ted. In Freudian terms, Mort is the Ego, his duplicate is the Superego and Shooter is the Id.
Throughout the movie, whenever Mort is seen arguing with his ex-wife or her new boyfriend in public, the people who sympathize with Mort are always strangers (such as the lawyer who they speak to after the house burns down). The people who already know Mort are never very sympathetic, because they know how selfish and mentally unstable he is.
