How was this serial killer also able to have a family?

I watched an episode of 48 hours about a serial killer named Israel Keyes who was apprehended in 2012. He was a horrible monster who killed at least 12 people BUT he also was a loving father and boyfriend. It doesn’t make any sense. How could he switch between two drastic modes? He also had a job and friends but in secret; he killed people, raped them and buried their bodies in lakes. He genuinely loved his daughter and after she was born, admitted to investigators that he could no longer kill children. He displays many signs of psychopathy yet he seemingly had an island of humanity in a sea of evil. What is about his brain that causes the literal Jekyl and Hyde? Is it something neuroscience can’t explain yet?