John Hinkley is a good example.
Andrea Yates is another.
I think both were correctly judged insane.
I do not believe JH was. He stated in his own words...he was obsessed with killing people. His planning and other statements tell me that he was not acting as a Schizophrenic. Or that he didn't know that what he was planning is wrong.
I believe both of the above people did have those issues. I have not heard anything that tells me that he had any issues with continuing on with his regular life while implementing his plan. There is no "slipping in and out of reality" here. Or if there was, his plan was not included in his "out of reality" part. He was going about his daily life, including taking college exams (though he didn't do well...which could have been because he was planning to commit mass murder instead of studying) all the while planning all of this. Those who he has interacted with during all of this - during his purchases etc. - that say he was not acting out of character. Even if they didn't know him, they could see he was acting within "society normal".
From my understanding, people who are actually insane have a hard time taking care of themselves, normal ADL's. He clearly had no issue with this, while putting together a very detailed, organized plan. He wasn't impulsive, otherwise he would have just taken the gun and shot up whatever the first place he got to.
Those are my thoughts...hope this helps?