Snipped this, respectfully.
Look, here's the issue with the word evil. In your mind, you seem to have assigned it a very simplistic meaning. Not putting words into your mouth here, just paraphrasing what you said. From my standpoint, it's something immensely complex and layered.
Mental illness is a real, rather common, malfunction of an area of the brain. There's lots of causes and lots of types, but the bottom line is that it's an actual, medical condition. Just like if someone lost part of the function of their liver, for example. It might have happened because of physical trauma to the liver, might be because of a virus or disease, might be genetic, or it might just be the way the liver was formed in utero. Mental illness is the same, from what I understand of it. It can be caused by a number of factors - biological like pre-natal damage, genetics, even a virus that's affected part of the brain, substance abuse. But then there is also this other layer of psychological and environmental causes.
I'm definitely not a person who is attempting in any way to deny or downplay the prevalence or importance of having a legitimate mental illness. I have, however, mentioned that I was concerned that because many diagnoses of mental illness are made on the basis of empirical evidence, they are somewhat subjective. If something is wrong with a liver, there are blood tests and scans and other tests to rule out some causes and confirm others. There are such tests available for some types of MI as well, but they are not commonly used outside of research studies for most diagnoses, to my knowledge.
I'm not a mental health professional, although I have some training in recognizing symptoms and experience with a variety of therapy options and behavior modifications as a result of my line of work. So, some of the above might be incorrect or off a bit, I'm open to correction if so.
Now - all of this is to say that I don't think that EVERYONE who kills has a mental illness. For sure, our prisons are rampant with people who do have either diagnosed or undiagnosed mental illnesses, and the link between the two is clear. No denying that. But, just as not everyone with a mental illness becomes a murderer (not even close), not everyone who is a murderer has a mental illness.
That's where some of us may differ. Some people think that anyone who kills clearly has something wrong (physically, chemically, etc.) with their brain that made them kill. I believe that's a bifurcation fallacy, because not killing is a societal norm, not an innate human function. In fact, the imbalance of power hypothesis of evolution states that we as humans are predispositioned for warfare. Societal factors build the resistance.
I'm getting way off topic here, so let me leave you with this question - do you believe that everyone who kills has a mental illness?
I've never seen anything that proves it to be so. If a study came out today that said every person who took a life has a diagnosed mental illness, then okay. I'd have to change my mind. I've been wrong lots of times and I don't see that stopping any time soon.
Because I don't believe all murders are mentally ill, I have to recognize that sometimes, people who have no brain-function related reason to kill choose to do so. Evil is the most recognized word that our language has for those people, in my opinion. We could call them something else, something that doesn't seem so trite, played out, or religious. I just don't know what that something else would be, you know? Societal norm breakers?
Rather than being simplistic, I think it's more complicated to look at someone without a mental illness and figure out what caused them to kill.
Just my way of looking at it. There's a possibility that both these girls have a mental illness. But, there's also a possibility they don't.