In a case study of criminalized behaviors, how do you define someone criminally insane?

The context is based on a particular setting and of course the theme is: what constitutes someone criminally unfit or insane. Where do you decide that someone isn't fit based on what they have done, if you could use the same bias judgment upon all those that have committed a inhumane act on society. But one is fit, and the other is not, what is the defining guidelines that separate the two into different groups.