As said, there are a myriad reasons (not excuses) as to why this might occur, be it lack of biology between the child and parent, jealousy, substance abuse, anger etc etc. Rather than trying to look at it atomistically, you could view it holistically, in three ways:
1. Disinhibition: socio-cultural and situational pressures may cause torturers to undergo a lessening of moral inhibitions and as a result act in ways not normally countenanced by law, custom and conscience (or societal acceptability).
2. Organisationally: like many things, once torture or abuse becomes established as part of internally acceptable norms under certain circumstances, its use often becomes institutionalised and self-perpetuating over time, as what was once used exceptionally for perceived necessity finds more reasons claimed to justify wider use.
3. Dehumanisation: this is the point reached whereby the torturer/abuser, see victims as objects of curiosity and experimentation, where pain becomes just another test to see how it affects the victim.
Again as said, you are trying to view the rational aspect of an irrational mind � which is the science of psychology � and one many of us cannot grasp. Similarly any abuse and torture (physical, sexual and psychological) of children, or indeed humans in general is a repugnant human trait to many of us, and one we could never knowingly understand nor comprehend.