Is there a major difference between having something seriously wrong with you (mentally or behaviorally) and being insane?
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.