I'm conflicted.

No, I don't think anyone should be "arrested", ever. (Abolish the police!)

However, idiots who say things like that kid did-- or those shooting off their mouths about Koch-- need to face consequences. It's not OK to get caught up in the moment and make credible threats-- but I can't know if the kid's threats were credible. I don't believe the Twitterheads were making credible threats-- that looked like virtue flashing to me. But their tweets could lead mentally unstable people into believing it's OK to attack and kill.

To me a credible threat is when, to the best of your knowledge, the threatener has the intention to follow through with the threat and the means to actually carry it out. I don't think cops/the "justice system" are qualified to figure it out.