Being locked up because you are insane and a threat to other is being locked up. The difference is there is no need to keep you in jail for a set amount of time

Being locked up because you are insane and a threat to other is being locked up. The difference is there is no need to keep you in jail for a set amount of time as a deterrent. They keep you in the institution until you have been declared sane. Which could be much less than the jail term for a similar crime or until you die. You probably have "not of sound mind" and "temporary insanity" confused.