Imagine, for a moment, that you are driving at 30 miles per hour through a town with a clearly posted speed limit of 35, and are pulled over for speeding. When you object that you were going well below the posted speed limit, the cop points to one of the signs, which now declares that the speed limit is 15 mph. A few seconds later, the sign changes to 20. Then 10.
Or scratch that -- imagine you've been pulled over for driving 40 miles per hour in that 35 mph zone. You're busted. Fair cop. And the town ordinances specify a fine of $50 for that offense. But when you schlep down to the courthouse to pay the fine, the clerk sees that you have red hair and decides she'd rather throw a dart at a board on the wall to determine your actual penalty, which may now range from "get out of jail free" to "death by lethal injection."
Setting aside the fact that Alyssa Bustamante's crime wasn't speeding but instead the brutal killing of a nine-year-old girl, the situation is very similar.
Bustamante was 15 years old at the time she committed the crime. She was clearly a "juvenile" under Missouri law, and that law specifies how juveniles are to be treated.
But Missouri law has hooks in it allowing (when it's politically advantageous) for "juveniles" to be miraculously re-classified as "adults" for purposes of criminal prosecution.
Arbitrary legal age classifications are stupid and evil enough on their own without throwing in capricious exceptions to those classifications for use by prosecutors and judges who may very well be thinking more about re-election or promotion than about the actual facts of the cases they try.
Keep in mind here that nobody came to Alyssa Bustamante the day before she murdered that little girl and said "I wave the magic wand of adulthood over you. You may now drive, vote, buy alcohol and have sex. Oh, and by the way, if you commit a crime, you'll be held fully responsible for it."
No, the thing was ex post facto: "At the time you committed the crime, the potential penalties were X. But because the newspapers love a good 'tough on crime' story, we've decided to change the penalties in your case to Y. Have a nice day."
That idea, given its wide application, is at least as evil as stabbing and strangling a nine-year-old to death.