"Too mentally incompetent to stand trial" should mean "too mentally incompetent to live in society at all". Since "not mentally competent" isn't a justification or declaration of a court but rather a declaration of the person being accused, it's entirely a voluntary procedure, and should result in Compassionate Canadian Healthcare. If you're sane, you should stand up there, declare your guilt and sanity, justify your situation as to what punishment you believe you deserve, and do your time. Or declare your non-guilt, and demand evidence be provided that you performed any wrong-doing. You know, court stuff.

"I'm fuckin' nuts, I kill people for fun!" -> "Sounds legit, you're free to go! Not guilty since you're nuts! Have fun back on the street!" should never be, in any sane society, the actual process.