i don't expect a brain 'malfunction' would show up on autopsy. A brain abnormality might however.
If, by a brain malfunction, you mean an abnormal response then I think that's entirely possible. Either, through some incidence in their past or some malfunction of their pleasure response centre of the brain, these individuals actually find pleasure in other people's pain and misery. There are quite a few people like that, though to a lesser degree than torture and murder.
When the pleasure centres of the brain are stimulated, usually by experiences such as food, sex, laughter, alcohol, smoking, drugs, etc. we seek to reproduce these pleasures. Part of the reason some people overeat it is, I'm sure, this pleasure response. People who overeat are in control of themselves and could override that response, in the same way a smoker could choose to give up.
Now for some reason in people like Brady, it is the torturing of children that stimulate that response so he sought to replicate that pleasure. Deliberately.