It doesn't really make it hard to participate in society. Mental deficiencies are not a choice. One reason to break the law is not caring about the law.
Do you think people are going to trade and interact equally with violent criminals as with peaceful people?
Yes, they are two separate classifications. You can be a bad person without any mental illness. It is more common to be a bad person and same then a bad person with a mental illness.
So evil vs insane? How do you tell the difference between a person doing evil deeds because he is evil and a person doing evil deeds because he is insane?
You're just repeating the classification. I'm asking how do you tell the difference? People don't have labels on them displaying their mental classification.