He deserves his day in court though. He never should have been in that situation and the way he acted seemed to be akin to that of a kid who thought he had it figured out, most certainly misguided or poorly advised by others.
So, in terms of mercy, if he is to receive any; it will be due to extraneous circumstances. He was well armed yes, shouldn't have been there yes, but he was placed in a vulnerable position. I don't know the laws but this might be enough in his state.
I can't imagine being as bold as he was at 17. It can only come from some form of reinforcement, like someone watching Dirty Harry or Charles Bronson and running out to clean up the streets while his acquaintances cheered him on. It's a horrible way to view the world and civil liberties, but some are lead down a path they shouldn't have gone down.