Second Trip — Robert Silverberg

Science fiction novel set in the year 2011 (about 40 year when book was written). Because capital punishment has been deemed too harsh, violent criminals are subjected to coercive therapy that effectively erases their memories and replaces them with artificially constructed memories to form a new personality deemed useful to society. Protagonist Paul Macy, a rehabilitated criminal, finds his original personality, a serial rapist named Nat Hamlin, reasserting itself in defiance of the mental conditioning.