Or, go ask the voters of the State of Oregon. They have a different interpretation, which is why they've legalized assisted suicide.
Can the mentally ill consent?
No.
No, it takes much more than consent on the part of the murdered, if you'd bother to check. The state sets all manner of legal hurdles in front of you before you're allowed to kill someone who consents to be killed.
Yet another case of the state's intrusion into our lives, huh? ;-)