Oh yea of course, there are always cases that are clear black-and-white, but the same could be said about due process. If fifty people watch a guy stab someone

Oh yea of course, there are always cases that are clear black-and-white, but the same could be said about due process. If fifty people watch a guy stab someone to death, a police investigation and full trial probably isn't quite as necessary; that doesn't mean due process shouldn't exist, because the job of determining the boundary for "obviously guilty (or obviously insane, in the original example)" needs to have some formalism around it. I'm not even saying that the government _shouldn't_ be involved in assessing mental health, just that people are wary of it for a good reason (in the same way that people are wary of any form of testing as a requirement for voting, because of the way that it was so horribly abused in the past).