First of all, can I say that I believe that they each had damaged personalities before they met, but the explosive evil was unleashed when they got together. As two individuals, I think they were far less dangerous than as a couple.
Don't get me wrong with what I say next - I am not for one minute likening all relationships with loved ones and friends etcetera with that of Brady & Hindley's relationship.
I think that in any relationship, whether it be romantic or friendly - you can relate to and react with certain people in a certain way and with a different person in a different way.