You can be very mentally ill and meticulous and organized at the same time. It is a fallacy to believe otherwise.
If you research personality disorders (Yes, they are a mental illness, diagnosable using the DSM-V, are evidenced based with specific criteria). Mental illness is not always visible or obvious, especially with personality disorders. Any Mental Health Professional will tell you that. Not all Mental Illness are treatable with medications (Personality Disorders a definitely not) and do not present with the same symptoms of obvious acute psychosis and disorganized behavior.
Although Borderline Personality Disorder can sometimes mask itself and look a lot like Bipolar Disorder. A good clinician will be able to tell the difference over time.
Again, to say someone who commits such a horrific act like this is "Mentally Ill" is
not an attempt to excuse the shooter. There is ALWAYS a component of mental illness and or diagnosable personality pathology involved in people who commit these acts. That does not mean that they did not know what they were doing was wrong at the time. In fact, certain personality disorders would ENJOY it.
He was homicidal and suicidal. That right there is the exact criteria that will get you psychiatrically hospitalized against your own will
legally if it is detected and you wind up in front of a psychiatrist and mental health team for assessment.
To say that meticulously planning and carrying out a massacre undetected somehow indicates that you are not at all mentally ill is to completely minimize the severity and persistence of personality disorders. A person that is operating at optimum mental health would not even be
capable of doing something like this. Their empathy and compassion as a human being would stop them.
Also, there is a strong correlation between pathological gambling (SP was a gambling ADDICT, just with means and good at what he did) and Cluster B personality Disorders in established mental health research.
Cluster B personality disorders are diagnosable mental illness that are difficult to treat, but nevertheless are mental illnesses.There are entire treatment models used to specifically treat them, namely Cognitive Behavioral Therapy (CBT) and Dialectical Behavior Therapy (DBT) (most successful with Borderline Personality Disorder. Antisocial Personality Disorder is more treatment resistant but can in some cases respond or spontaneously remit, albeit rarely. Cluster B Personality Disorders include:
Antisocial Personality Disorder (Formerly sociopathy in the professional vernacular or Psychopathy)
Narcissistic Personalty Disorder
Histrionic Personality Disorder
Borderline Personality Disorder.
I provided a peer reviewed journal article on this in an earlier another thread. It is also important to note that you can have a personality disorder AND another type of mental illness at the same time, such as Bipolar Disorder etc. It is not always one or the other.
Here is some literature on the prevalence of personality disorders in pathological gamblers , especially antisocial personality disorder .
This is just one published study of many and I chose this one at random.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2593739/