I think it's odd how people are seemingly unable to rationalize the mind of a psychopath. All men are capable of evil, we are all two faced. In order to live in modern society, you quickly learn how to slip into different sociological roles, the student, the sibling, the friend, the son, etc. It's not as though any of them aren't you, but none of them are fully you either. We all have a darker side that never sees the light of day, a dark side that has kept our race alive for thousands/millions of years by making it ruthless.
Granted, I'm not familiar with this particular person (BTK), but keep in mind that many psychopaths go into a panicked denial when they learn they are psychopathic, some never actually accepting the diagnoses. Also keep in mind that an estimated 10% of the population could be considered psychopathic, unable to feel empathy towards others.
Psychopaths create their feelings for others; that does not mean they are fake. There is simply a few layers of learned and self-taught mental work that they need to go through when processing things. Instead of seeing his daughter being bullied and feeling angry, the psychopath sees his daughter being bullied, processes the situation, subconsciously reminds himself that he needs to protect his daughter, that he will be seen as a bad parent by others if he doesn't, and then he feels angry and wholeheartedly thinks it's directly for his daughter's safety, when it's a far more involved train of thought.
tl;dr I don't think he shielded his family from himself, and he probably cared about them quite a bit. His mind simply fell into a dark place and his mind was far too suited to the darkness so he became addicted. He would never have committed those acts on his daughter or those he cared about. For a psychopath, it's nothing more than a form of fucked-up artistic expression.