Insights into killing can be obtained by considering theories of general crime and non-lethal sexual offending. This chapter considers the theoretical positions of Marshall, Barbaree, Malamuth, Ressler et al. and Seto. Drive theory fails to explain behaviour and has largely been replaced by incentive motivation theory. The distinction between organized and disorganized lust killers is not an absolute one but represents two extremes on a continuum. By sensory preconditioning, two events that occur together can become associated, such as sexual desire becomes paired with an aversive emotion. Even though killing appears to be maladaptive, evolutionary approaches can still give insights. The theory of Belsky et al. suggests that uncertainty of social support during development is assimilated and plays a role in determining reproductive strategy. An evolutionary argument suggests that male serial killers reflect a hunter/stalking strategy, whereas female serial killers reflect a strategy of staying at home and maximizing genetic benefit.