Anyone can claim they have mental issues. I'd say it's fair to say that most people have mental issues. But it doesn't mean that they should be let off because they have issues.
"I do not underestimate the ability of fanatical groups of terrorists to kill and destroy, but they do not threaten the life of the nation. Whether we would survive Hitler hung in the balance, but there is no doubt that we shall survive al-Qa'ida." -- Lord Hoffmann
And if they wound/kill someone they go to specialised courts (I don't know how you call it, here it is the "judge for youth") that takes special measures because at a low age there are still chances to re-educate them and change them. Sending them to jail is not the appropriate measure to take.
As for the crazy/retardeds...who commit crimes, a jail is not the appropriate place to them: they are ill and should be treated. When they are dangerous they aren't allowed to get out of the psychiatric center, but it's not a "penal measure", it's not a "jail". The result is the same (they don't get out) but the goal (punishing vs healing) and the measures (jail vs psychiatric center) are different
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Jon Huntsman for President
Someone who does something like this is empty and devoid of any emotions. Kinda reminds me of something from a horror movie and like something Micheal Myers would do