May I ask in what way it seems obvious? His defense didn't even make this claim.
It's murky waters to me, this phenomenon some people have of obsession with violence and acting it out. Regarding what that is -- Is it mental illness or rather a personality disorder? If EC had MI, why wasn't it part of his defense?
In many states, there's no reason to bring mental health into the case unless there's the possibility of an insanity defense. Some of the major personality disorders are due to brain issues/genetics. Let me just say that the parents appear to be in a category where psychiatrists and psychologists cannot completely agree on what to call it (DSM has one set of criteria and one label; the actual research on the psychological and neurological/brain issues use different language - language including words like "sociopath.")
This murderer shows the signs of sociopathy - and even the classic "flight to health" when caught. The first line of treatment for such people is consequences. His apparent acceptance of the consequences is also classic for a sociopath. They know when to fold their cards. He will remain a sociopath in prison, IMO, but he'll be amongst his own kind - for many people in prison are sociopaths.
It's my view and the view of many psychiatric researchers that such persons are not randomly created. They have underlying brain issues, and it runs in families (as we see here, IMO). DSM calls it "Antisocial PD," but I don't know anyone in the psychiatric professions who believes that there's not a biological/brain basis to it as well (via brain analysis, such as S.P.E.C.T. or PCL-R analysis).
Here's one article about the re-emergency of the term "sociopathy" and its cousin term, "psychopathy." It focuses immediately on the brain differences between sociopaths and normals, you don't have to read too far - and since it was published, even more differences have been found.
The debate has gotten clearer in the past 10 years (more agreement over the use of those two terms - both of which are mental illnesses IMO - but of course everyone is free to view it some other way). Sociopathy and psychopathy are not normal brain states. Having abnormal brain structures such as a deformed amygdala is, by definition, an illness in medicine (IMO). The article doesn't go into the genetics, but there are hundreds, if not thousands, of academic articles on that topic as well.
Many criminally minded people do not want to use an insanity defense (many reasons, but one is that some prefer prison to hospitals for the mentally ill - and having worked in both places, I get why they are of that viewpoint). But one other big reason is that the legal definition of insanity has nothing to do or little to do with the medical definition - which is what I'm talking about. It's unlikely that this defendant would meet the legal standard for insanity in most states. Merely having a mental illness is not the same thing as being criminally insane.
IMO.