What he did is what all teenagers do. Bitch to friends, say shocking and outrageous things, argue with anyone, feel all alone/emo angst like. The biggest difference is we now have this platform to say anything we want at any time with 'anonymity' and we don't think about how it comes out.
I was a kid like him. Hard working, never been in a fight, also thought of suicide as a teenager (like most of them) and i definitely said dumb things like 'Oh i'm gonna kill this person!' Or "god these kids should get hit by cars"
I still say thinks like 'I hate ___! I just wanna kill her/him/it!' or 'F this place let it burn!' That doesn't mean I am going to follow through with any of that. It is just venting frustrations or annoyances.
The police should never have arrested him and held him for so long in Jail. If anything respond to something like that and get a search warrant. If there is no sign of planning, or anything that shows intent...let them go with a fine for being a dumb ass. Or something else along those lines. Max time allowed to hold someone...48hrs at the police station, not a prison with actual Criminals.
Besides I am fairly sure that if someone was planning on shooting up a kindergarten, or bombing something they wouldn't post it on line for everyone to see.