When a person commits suicide after a heinous crime, they realise what they have done and cannot face either the punishment or the public, after it, so they kill themselves. I think for some people who are mentally ill they are coming out of a trance almost and they realise the way they have acted is unacceptable, where as a person of pure evil would not batter an eye-lid.
The thought processes of a normal person would be very different to that of a healthy person, for example a healthy person should be shocked at the events of September 11th 2001 but a mentally ill person may find it funny or amusing, this is the type of thought which could lead them to murder someone.
Finally the most blatantly obvious reason, they have a history of mental illness in their family or the person has. This explains itself and people who have history are usually kept under close watch.
The above reasons support that some people are not evil and mentally ill.
In the next paragraph I am going to give reasons as to why I believe some people to be pure evil, these are: -
Crimes are committed perfectly and are planned perfectly
The person is a genius
The crimes are normally on a far greater scale
They have committed crimes in the past
There is often a motive
The crimes that are committed in a perfect way are planned and worked out so that nothing can go wrong, for example the September 11th terrorist attacks. That attack took months if not years to plan and was devastating when executed, killing thousands while the leader did not fell any remorse or guilt for what he had done, now that is pure evil.
That was a huge crime, Osama Bin Laden killed thousands of people, in the same way that Adolf Hitler the leader of the Nazis killed thousands, nearly millions in the war, a person who is mentally ill could not plan such an attack.
Also only a person of extreme intelligence could devise a battle plan like Hitler did, so his brain had to be perfect, with no mental illness to think as well as he did.
People like Bin Laden and Sadam Hussain are known terrorists and have committed many crimes in their time, so they do not break down emotionally when they kill someone, like a mentally ill person would do, an evil mind has no emotion.
Finally for most terrorist attacks there is a motive, for world war two there were many motives. But a mentally ill person often picks someone a random with no reason to kill, rape or assault them e.g. the Holly Wells and Jessica Chapman case, the offender had no reason to kill them, what had two ten year old girls ever done to him and her?
I conclude that some people are mentally ill and can prove it, but others use has an escape route to cover up what they have done wrong!
