Martin was given plenty of opportunities to change, but he didn't. Instead of being thrown in JDC for his theft of jewelry, the stolen jewelry was re-classified as "found property," so that it wouldn't be reported as a crime (Baker Act, IIRC). Instead of being sent to JDC for assaulting the bus driver, he was given a light slap on the wrist.
The autopsy report showed liver damage that was consistent with what you see when someone abuses dextromethorphan. While it's relatively harmless in the proper doses (a couple tablespoons of cough syrup at the most), it's also well-known that DM acts as a euphoric drug when given in higher doses, and even hallucination / psychosis.
http://en.wikipedia.org/wiki/Recreational_use_of_dextromethorphan
It was also known that Martin was actively seeking to make purple drank, and that he had been a user of the concoction.
http://theconservativetreehouse.com...-drug-use-culminates-in-predictable-violence/
Edit: Sorry about the poor attached image. The link shows the image in its proper size, and easy to read.
Somehow I can't help but to think that all of this was 'overlooked' by Al and Jesse.
*Listens closely to the sound of a broom sweeping something under the rug*