Alcoholism causes brain damage in the alcoholic. If the alcoholic stops drinking, the damage doesn't go away, but the brain does what it can to create new pathways to restore functioning.
While the alcoholic is still drinking, though, their thoughts are distorted. Their behavior changes. Some alcoholics become mean and loud and obnoxious, even violent, when they drink. Others kind of get quiet and maudlin and feel sorry for themselves and victimized by the world.
As Earthworm said, the "split personality" goes with the territory. Real recovery brings out who the person REALLY is.