It’s possible that Jeremy Bamber either doesn’t realise he’s a psychopath or refuses to accept it.
He showed huge signs of psychopathy when sat with the police outside the farmhouse: his general manner was calm, collected, and he even spoke several times about buying a Porsche. No normal person who’s terrified that their whole family are in mortal danger would even think about sports cars...
He was so calm, in fact, that when one officer later arrived on the scene he mistook JB for a detective: such was his calm manner.
It’s known that June accidentally dropped him on his head when a baby, and I think (though it isn’t corroborated) he suffered a fractured skull. It’s a fact that head injuries can cause psychopathy, and when Jeremy failed his medical examination to train to become a registered scuba diver in NZ, in the hope of teaching, he blamed Sheila for dropping him on his head.
In some ways you have to pity him if that accident caused his psychopathy, the whole thing is tragic all round. But even psychopaths, despite their lack of empathy and their warped emotions know right from wrong. He isn’t unintelligent or suffering from mania, so however devoid of emotions he may be, he would have known it was wrong to kill all his family — but he didn’t care.