I dislike that in the article they removed the context of his statement. Just reading this, it sounds like he was making a threat, but if you read the KHOU.com (Houston) article it appears that here's merely mocking the person who is calling him insane.
NPR version:
"I think Ima shoot up a kindergarten / And watch the blood of the innocent rain down/ And eat the beating heart of one of them."
KHOU version:
'Oh you're insane, you're crazy, you're messed up in the head,’ to which he replied 'Oh yeah, I'm real messed up in the head, I'm going to go shoot up a school full of kids and eat their still, beating hearts,’ and the next two lines were lol and jk.,"
http://www.khou.com/news/texas-news/Texas-teen-charged-with-...
NPR version: "I think Ima shoot up a kindergarten / And watch the blood of the innocent rain down/ And eat the beating heart of one of them."
KHOU version: 'Oh you're insane, you're crazy, you're messed up in the head,’ to which he replied 'Oh yeah, I'm real messed up in the head, I'm going to go shoot up a school full of kids and eat their still, beating hearts,’ and the next two lines were lol and jk.,"