If 'evil' actually exists how would you define it? Is it someone who knows right from wrong deliberately doing something harmful to another sentient creature, and having no conscience about it? If so, that behaviour already has a name, sociopathic, or sometimes, psychopathic, behaviour.
Is 'evil' just shorthand for something we each instinctively understand in our own way to be very wrong and damaging to others, a descriptor that fills a need not otherwise Identifiable? If so, it's the same as religious belief which also has no intellectual or factual foundation for it, so arguing will never settle the point as it never has between people, and nations, of differing religious beliefs.
Re my previous post, I left my ex-husband long before he committed a really serious crime and was formally diagnosed as psychopathic (36 on the Hare PCL-R checklist) but had interviews with the forensic psychiatrist to put my experience of his behaviour with all the others' so he could finally be assessed as to whether he should go to prison or a secure hospital.
If behavioural scientists are right and sociopathic behaviour is due to abnormal brain chemistry or structure, it raises the possibility that sufferers could end up being treated and able to live a normal life, having a conscience like everyone else. How many women (they were mostly women) were burned at the stake because of behaviour that could have easily been attributable to eating mouldy bread - ergotamine poisoning causes hallucinations and other frightening behaviour. Women (mostly) now thought to be migraine sufferers were incarcerated in 'lunatic asylums' because of what was called dangerous behaviour when they couldn’t behave ‘normally’ during a migraine. It’s well known now what migraine is and it runs in families. Wasn’t T S Eliot’s first wife, Vivienne Haigh-Wood one of these victims, although she had gynae problems as well that could have affected her emotionally?
It also raises the possibility, though, that we might go back to an age where people are subjected to lobotomies and similar surgery, against their will, and all the horrors that accompanied that.
Maybe a future Karen Matthews might get a diagnosis and treatment that could prevent criminal behaviour, make her better able to cope with her own life, and give her children the better life they deserve.